stop hovering when the cursor points at no tile

A cursor resting on a panel or outside the window kept whatever it last
pointed at: the ghost, the tunnel preview, the deconstruct tint all stayed
put, because "not hovering" was not a state the build mode could hold. Make
both ghost tiles optional, clear the hover with them, and re-derive it when
the cursor comes back or a mode is entered.

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

docs/requirements.md

@@ -123,7 +123,7 @@ Any ship, module, building, or assembler recipe id that appears in no unlock gro
 - REQ-BLD-QUEUE: Placed buildings enter a construction queue and are built one at a time. Each building takes a duration defined in `buildings.toml [[building]].construction_time_seconds` to construct.
 - REQ-BLD-ASTEROID-ONLY: Buildings can only be placed on asteroid tiles (per surface_mask; tiles marked `S` may extend into space).
 - REQ-BLD-BUILDER-MODE: Clicking a build button activates builder mode for that building type. Builder mode is exited by right-clicking in the game world or clicking the same build button again. (Exception: while a belt drag placement is in progress, right-clicking cancels that drag instead of exiting, and builder mode stays active — REQ-BLD-BELT-DRAG.)
-- REQ-BLD-GHOST: While in builder mode, a ghost of the building is rendered at the tile under the cursor, showing where it would be placed. The ghost follows the tile the cursor points at, which includes the view scrolling (REQ-UI-SCROLL) under a cursor that has not moved: the ghost then moves across the world with the scroll rather than sticking to the tile it was last placed on by a mouse move, exactly as a running selection box does (REQ-UI-MULTI-SELECT). The same holds for everything else the hovered position determines — placement validity (REQ-BLD-PLACE-VALID), the tunnel end being placed (REQ-BLD-TUNNEL-MODE), a belt drag's path (REQ-BLD-BELT-DRAG), the blueprint ghost (REQ-UI-BLUEPRINT-MODE), and the deconstruct hover (REQ-UI-DECONSTRUCT-BUTTON). A cursor that is not over the game world — resting on one of the floating panels, or outside the window — hovers nothing, so scrolling leaves that state untouched (REQ-UI-CONTROLS-PANEL). The ghost is drawn semi-transparently in the building type's own visuals — its `fill` and `outline` colors and `glyph` from `visuals.toml` — so that different building types are visually distinguishable in builder mode rather than all looking alike. When the current cursor position is invalid, the ghost instead uses the distinct "invalid" color (REQ-BLD-PLACE-VALID), which overrides the per-building coloring.
+- REQ-BLD-GHOST: While in builder mode, a ghost of the building is rendered at the tile under the cursor, showing where it would be placed. The ghost follows the tile the cursor points at, which includes the view scrolling (REQ-UI-SCROLL) under a cursor that has not moved: the ghost then moves across the world with the scroll rather than sticking to the tile it was last placed on by a mouse move, exactly as a running selection box does (REQ-UI-MULTI-SELECT). The same holds for everything else the hovered position determines — placement validity (REQ-BLD-PLACE-VALID), the tunnel end being placed (REQ-BLD-TUNNEL-MODE), a belt drag's path (REQ-BLD-BELT-DRAG), the blueprint ghost (REQ-UI-BLUEPRINT-MODE), and the deconstruct hover (REQ-UI-DECONSTRUCT-BUTTON). A cursor that is not over the game world — resting on one of the floating panels (REQ-UI-CONTROLS-PANEL, REQ-UI-BUILD-BAR, REQ-UI-SELECTION-PANEL), or outside the window — points at no tile and therefore hovers nothing: no builder ghost and no blueprint ghost is drawn, no tunnel connection is previewed, and no building is tinted as the deconstruct hover, until the cursor returns to the world. The mode itself is unaffected — the player is still building, just not over anything — and the position the ghost had is not remembered: it is re-derived from wherever the cursor comes back. This applies whenever the cursor points elsewhere, including at the moment a mode is entered, so a mode entered from a build button (REQ-UI-BUILD-BAR) shows its ghost only once the cursor is over the world, while one entered by hotkey (REQ-UI-HOTKEYS) under a cursor already there shows it at once. The one exception is a gesture that holds the mouse button — a belt drag (REQ-BLD-BELT-DRAG) or a selection or deconstruct box (REQ-UI-MULTI-SELECT, REQ-BLD-DECONSTRUCT-BOX) — which goes on following the cursor across the panels and beyond the window until the button is released. The ghost is drawn semi-transparently in the building type's own visuals — its `fill` and `outline` colors and `glyph` from `visuals.toml` — so that different building types are visually distinguishable in builder mode rather than all looking alike. When the current cursor position is invalid, the ghost instead uses the distinct "invalid" color (REQ-BLD-PLACE-VALID), which overrides the per-building coloring.
 - REQ-BLD-ROTATE: While in builder mode, pressing Shift+R rotates the ghost 90° clockwise and R rotates it 90° counter-clockwise. Rotation affects the direction of the output port.
 - REQ-BLD-PLACE: Clicking a valid tile in builder mode places a construction site and adds it to the build queue, consuming building blocks from the global stock. (For belts, placement is instead deferred to a drag gesture and happens on mouse release — REQ-BLD-BELT-DRAG.)
 - REQ-BLD-PLACE-VALID: A placement position is valid only if (a) every footprint cell in the rotated `surface_mask` is satisfied by the underlying terrain — `A` cells coincide with asteroid tiles, `S` cells coincide with space tiles — (b) no footprint cell overlaps an existing placed building or construction site, except as allowed by REQ-BLD-ROTATE-IN-PLACE (builder mode) or REQ-UI-BLUEPRINT-OVERLAP and REQ-UI-BLUEPRINT-TRANSFER (blueprint placement mode), and (c) the player has enough building blocks to afford the building. The ghost (REQ-BLD-GHOST) is rendered in a distinct "invalid" color — overriding its per-building coloring (REQ-BLD-GHOST) — when the current cursor position fails any of these conditions.