clear the selection with Q, and drop it on entering a build mode

Implements the requirements committed in 731b887.

Q becomes a three-way branch in the action table, which is the layer that owns
what an input does: ExitMode while a mode is active, the new ClearSelection
while something is selected, EnterDeconstruct otherwise. The three partition
the situations between them, so resolution stays first-match-wins over
available actions and the handler never re-derives the precedence -- which is
why ClearSelection gets its own event rather than joining ModeCancel on Q.

GameWorldView clears the selection at each of the three events that enter a
mode; those are the only ways in, whichever button or key the player used. No
clear is needed where ModeCancel falls through to deconstruct mode: Q resolves
to ClearSelection while anything is selected, so there is nothing left by then.

The Selection context's Q row reads "Clear selection", sits last as the row
that hands the context back does everywhere, and carries the exit badge
styling -- one key that backs out should look the same wherever it appears.
Requirements follow that last point in REQ-UI-CONTROLS-CARD and
REQ-UI-CONTROLS-CONTENT.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ne3mejABZoLWKLh8fgpM3x

docs/requirements.md

@@ -669,7 +669,7 @@ The controls panel tells the player which controls are available right now. It i
   - **Input.** Mouse events over the panel are consumed by the panel 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 panel does not exit builder mode (REQ-BLD-BUILDER-MODE) or cancel a belt drag (REQ-BLD-BELT-DRAG). The only control the panel itself offers is the header click that collapses and expands it.
 - REQ-UI-CONTROLS-CARD: **Card structure.** The panel is a card with two parts, top to bottom:
   - **Header** — always shown, and the panel's only interactive element (REQ-UI-CONTROLS-PANEL). It holds a colored context dot on the left, the context's name beside it in upper case, and, for contexts that define one, a **detail suffix** separated by a middle dot (`BUILD MODE · Assembler`). The name and detail per context are given in REQ-UI-CONTROLS-CONTENT.
-  - **Rows** — one per available control, shown only while the panel is expanded. Each row is one or more **key badges** on the left — the key or mouse button drawn as a small bordered chip — and a **label** beside them naming what it does. An action reachable two ways carries both badges in the same row (`RMB` `Q` — Exit placement) rather than occupying two rows. A row whose action leaves the current mode is drawn with the destructive badge styling, distinguishing it from the rows that act within the mode. No row is ever drawn greyed or otherwise disabled: a control the player cannot currently use is not shown at all (REQ-UI-CONTROLS-ACCURACY).
+  - **Rows** — one per available control, shown only while the panel is expanded. Each row is one or more **key badges** on the left — the key or mouse button drawn as a small bordered chip — and a **label** beside them naming what it does. An action reachable two ways carries both badges in the same row (`RMB` `Q` — Exit placement) rather than occupying two rows. A row whose action hands the current context back — leaving a build mode, or clearing the selection (REQ-UI-SELECTION-EXCLUSIVE) — is drawn with the destructive badge styling, distinguishing it from the rows that act within the context; to the player both are the one key that backs out, so both are marked alike. No row is ever drawn greyed or otherwise disabled: a control the player cannot currently use is not shown at all (REQ-UI-CONTROLS-ACCURACY).
 
   The rows that are live in every context (REQ-UI-CONTROLS-CONTENT) are shown last, under a divider and the caption `ALWAYS AVAILABLE`. This holds in every context including the General one, which has context rows of its own above the divider like any other, so the card is read the same way wherever the player is.
 - REQ-UI-CONTROLS-CONTENT: **Content catalog.** The control context follows from the active build mode and the selection alone. Build modes are mutually exclusive (REQ-BLD-BUILDER-MODE), so exactly one context applies at any moment:
@@ -684,6 +684,8 @@ The controls panel tells the player which controls are available right now. It i
 
   The Selection context's detail counts the selection and names its category (REQ-UI-SELECTION-CATEGORIES): `<n> buildings` for a building selection, `<n> objects` for a field selection, in the singular at a count of one. The Build and Blueprint contexts show **the same rows** and differ only in their header.
 
+  **The row that hands the context back is last.** Where a context has one — `Q` — Clear selection in the Selection context, `RMB` `Q` — Exit placement in Build and Blueprint, `RMB` `Q` — Exit deconstruct mode in Deconstruct — it is the final context row, below the rows that act within the context, and it carries the destructive badge styling (REQ-UI-CONTROLS-CARD). The General context has none: with no mode to leave and nothing selected, its `Q` row enters deconstruct mode rather than leaving anything, and is an ordinary row that happens to come last.
+
   **Always-available rows**, shown in every context:
 
   | Badges | Label | Shown |
@@ -706,9 +708,9 @@ The controls panel tells the player which controls are available right now. It i
   | | `LMB` drag | Select area |
   | | `Ctrl` `LMB` | Add / remove from selection |
   | | `Ctrl` `LMB` drag | Add area to selection |
-  | | `Q` | Clear selection |
   | | `C` | Copy to temporary blueprint |
   | | `Ctrl` `C` | Create blueprint |
+  | | `Q` | Clear selection |
   | Build, Blueprint | `LMB` | Place |
   | | `LMB` drag | Place belt line |
   | | `R` / `Shift` `R` | Rotate |