float the build buttons as a bar over the game world

Implements the REQ-UI-BUILD-BAR rewrite: the build buttons leave the side
panel column for a widget floating at the bottom center of the game world
view, as one non-wrapping row. BuildButtonGrid is renamed BuildButtonBar and
its QGridLayout becomes a QHBoxLayout; the side panel column is left with two
equal-height panels.

Buttons become icon-only. Each face is composed into a single pixmap per
QIcon mode - hotkey badge in the top-left corner, chip icon centered, cost
and block icon below - because a QPushButton holds only one icon. That
replaces the custom-painted BuildButton and lets Qt grey an unaffordable
button by swapping the pixmap. The building name moves into the tooltip,
which now always leads with it, and a missing chip SVG falls back to the name
in the chip's place.

The badge labels come from InputMapper, searched out of the same table the
key handler uses so a badge cannot claim a key that does nothing. The
Deconstruct button has no cost, so it shows its name there instead, and sits
last behind a gap.

Parenting the bar to MainWindow makes creation order the stacking order: it
lands above the world view and its vignettes and below the modal dim, and Qt
routes mouse events to it rather than the world, which is the whole input
clause of REQ-UI-BUILD-BAR at no cost.

Requirements are amended for the two things the mockup added: the hotkey
badge and the Deconstruct caption.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JcReq7hVk4KUPhTDKWAG7K

docs/architecture.md

@@ -95,7 +95,7 @@ Schematic drops: when an enemy station set is destroyed, the simulation generate
 
 All UI interactions — building selection, builder/blueprint mode transitions, speed changes, deconstruct mode, escape menu, layout dialog requests — are communicated via EventManager events rather than Qt signals/slots. Each event is a small struct inheriting `Event` (e.g., `SelectionChangedEvent`, `BuildingTypeSelectedEvent`, `SpeedChangeRequestedEvent`). Widgets register as `CombinedEventHandler` for the events they care about and emit events via `EventManager::sendEventImmediately()`.
 
-Bidirectional interactions use separate request/notification event types to avoid infinite recursion (e.g., `ExitBuilderModeRequestedEvent` from `BuildButtonGrid` → `GameWorldView`, vs. `BuilderModeExitedEvent` from `GameWorldView` → `BuildButtonGrid`).
+Bidirectional interactions use separate request/notification event types to avoid infinite recursion (e.g., `ExitBuilderModeRequestedEvent` from `BuildButtonBar` → `GameWorldView`, vs. `BuilderModeExitedEvent` from `GameWorldView` → `BuildButtonBar`).
 
 ### Reading Simulation State
 
@@ -124,7 +124,7 @@ Within a single simulation tick, subsystems run in this fixed order. The order i
 Three product targets plus tests:
 
 - `lib/` — simulation + config. Depends on Qt Core + Qt Gui, toml++, tinyexpr. No QtWidgets.
-- `ui/` — QtWidgets + `QOpenGLWidget` code: header bar, game world view, selected building panel, build button grid. Depends on `lib` and on Qt's OpenGL widgets module.
+- `ui/` — QtWidgets + `QOpenGLWidget` code: header bar, game world view, selected building panel, build button bar. Depends on `lib` and on Qt's OpenGL widgets module.
 - `app/` — thin `main()` that creates the simulation, the UI, and wires them together. Depends on `ui`.
 - `tests/` — Catch2 tests. Links only against `lib`.
 

docs/requirements.md

@@ -556,11 +556,12 @@ The screen is divided into two columns: a main column (75% width) containing the
 - REQ-UI-BUILD-BAR: All placeable building types are shown as a **single horizontal row** of buttons with no grouping and no wrapping, inside a widget that **floats over the game world view** (REQ-UI-WORLD-SIZE), horizontally centered and anchored at the bottom edge with a small margin. Tunnel Entry and Tunnel Exit share a single **Tunnel** button (REQ-BLD-TUNNEL-MODE) rather than one button each. The bar is sized to its buttons and re-centers whenever the set of shown buttons changes (REQ-LOCK-BUILDING) or the view is resized.
   - **Overlay behavior.** The bar occludes the strip of the game world it covers; the world view itself keeps its full extent and the view's scrolling, ghost rendering, and tile geometry are unaffected (the world is not inset for the bar). The bar is drawn above the pause and deconstruct vignettes (REQ-UI-PAUSE-BORDER, REQ-UI-DECONSTRUCT-BORDER), which keep their full 100-pixel bottom band underneath it, and below the modal dim (REQ-UI-MODAL-DIM), which covers the entire game window including the bar.
   - **Input.** Mouse events over the bar are consumed by the bar and never reach the game world: hovering it shows no builder-mode ghost at the tile beneath, and clicking it neither places a building nor changes the selection. Right-clicking the bar does not exit builder mode (REQ-BLD-BUILDER-MODE) or cancel a belt drag (REQ-BLD-BELT-DRAG).
-- REQ-UI-BUILD-COST: Each button is **icon-only with a cost**: it shows the building's icon (REQ-UI-BUILD-ICON) and its building block cost with the `building_block` item icon (REQ-UI-BLOCKS-ICON, REQ-UI-ITEM-ICON) in place of the trailing `Blocks` word, e.g. `2` then a small block icon. The building name is not shown on the button; it is shown in the button's hover tooltip instead (REQ-UI-BUILD-TOOLTIP). When no icon file exists for `building_block`, the cost is shown as the bare number. The Deconstruct button (REQ-UI-DECONSTRUCT-BUTTON) has no cost and shows its icon alone.
+- REQ-UI-BUILD-COST: Each button is **icon-only with a cost**, its face composed of three elements: the button's **hotkey badge** in the top-left corner, the building's icon (REQ-UI-BUILD-ICON) centered below it, and the building block cost centered under the icon, shown with the `building_block` item icon (REQ-UI-BLOCKS-ICON, REQ-UI-ITEM-ICON) to the right of the number in place of the trailing `Blocks` word, e.g. `2` then a small block icon. The building name is not shown on the button; it is shown in the button's hover tooltip instead (REQ-UI-BUILD-TOOLTIP). When no icon file exists for `building_block`, the cost is shown as the bare number. The Deconstruct button (REQ-UI-DECONSTRUCT-BUTTON) has no cost and shows its name as a text caption in the cost's place.
+  - **Hotkey badge.** The badge names the build hotkey that activates the button (REQ-UI-HOTKEYS), so the player can learn the shortcuts from the bar itself. It is rendered smaller and dimmer than the cost so it reads as secondary. A plain-digit hotkey is shown as the bare digit (`1`, `2`, `3`); a Shift+digit hotkey is shown with the shift glyph prefixed and no separator (`⇧1` … `⇧6`); the Deconstruct button shows `Q`. A button whose building type has no build hotkey shows no badge and keeps the same face size, so the row stays even.
 - REQ-UI-BUILD-ICON: Each build button shows an icon. Icons are SVG files loaded at runtime from `data/icons/buildings/` (a sibling of the config directory, read the same way as `visuals.toml`), one file per button named after the building's id (e.g. `belt.svg`, `reprocessing_plant.svg`). The shared Tunnel button (REQ-UI-BUILD-BAR) uses `tunnel_entry.svg`; the Deconstruct button (REQ-UI-DECONSTRUCT-BUTTON) uses `deconstruct.svg`. Each icon is a rounded colored "chip" bearing a white line glyph, the chip color following the building's fill color in `visuals.toml`. A missing icon file leaves the button showing its building name as a text caption in place of the icon, so the button stays identifiable in the icon-only bar (REQ-UI-BUILD-COST); it is not an error.
 - REQ-UI-BUILD-TOOLTIP: Each building-type button shows a hover tooltip consisting of the building name followed by the descriptive text defined for that building type in `buildings.toml` (the optional per-building tooltip field). Because the button caption is icon-only (REQ-UI-BUILD-COST), the name is always part of the tooltip; if a building type defines no tooltip text, the tooltip shows the name alone. This tooltip is distinct from the recipe/schematic selection tooltip (REQ-UI-SELECT-TOOLTIP). The Deconstruct button (REQ-UI-DECONSTRUCT-BUTTON) is not a building type and so has no config-defined tooltip; it instead shows its own refund tooltip defined in REQ-UI-DECONSTRUCT-BUTTON.
 - REQ-UI-BUILD-DISABLED: Buttons for buildings the player cannot currently afford are shown as disabled. A disabled button's icon (REQ-UI-BUILD-ICON) is rendered in a greyed variant, with its colored chip background recolored grey while the white glyph is retained.
-- REQ-UI-DECONSTRUCT-BUTTON: A dedicated **Deconstruct** button is shown in the build button bar (REQ-UI-BUILD-BAR), as the last entry of the row and **visually separated** from the building-type buttons by a gap (not a divider line), because it toggles a mode rather than selecting a building type. Clicking it toggles deconstruct mode on and off, equivalent to the Q deconstruct toggle (REQ-UI-HOTKEYS). The button is shown in a visually active/pressed state while deconstruct mode is active. The button shows a hover tooltip stating the deconstruction refund (REQ-BLD-DECONSTRUCT): that deconstructing a fully-built building returns `world.toml [world].refund_percentage` percent of its building block cost once deconstruction completes, and that a construction site removed before it finishes building is refunded in full. When `refund_percentage` is 100% both cases yield the same refund, and the tooltip is simplified to state the single refund percentage without distinguishing the two cases. Unlike the building-type button tooltips (REQ-UI-BUILD-TOOLTIP), this tooltip is not config-defined text but is composed from the refund percentage.
+- REQ-UI-DECONSTRUCT-BUTTON: A dedicated **Deconstruct** button is shown in the build button bar (REQ-UI-BUILD-BAR), as the last entry of the row and **visually separated** from the building-type buttons by a gap (not a divider line), because it toggles a mode rather than selecting a building type. Its face follows REQ-UI-BUILD-COST with two differences: its hotkey badge reads `Q`, and because it has no building block cost it shows its **Deconstruct** name as a text caption where the building-type buttons show their cost — so it is the one labelled button in the bar. It is therefore wider than the building-type buttons, which share a uniform width. Clicking it toggles deconstruct mode on and off, equivalent to the Q deconstruct toggle (REQ-UI-HOTKEYS). The button is shown in a visually active/pressed state while deconstruct mode is active. The button shows a hover tooltip stating the deconstruction refund (REQ-BLD-DECONSTRUCT): that deconstructing a fully-built building returns `world.toml [world].refund_percentage` percent of its building block cost once deconstruction completes, and that a construction site removed before it finishes building is refunded in full. When `refund_percentage` is 100% both cases yield the same refund, and the tooltip is simplified to state the single refund percentage without distinguishing the two cases. Unlike the building-type button tooltips (REQ-UI-BUILD-TOOLTIP), this tooltip is not config-defined text but is composed from the refund percentage.
 
 ### Blueprint Panel