use WorldCoordinates in ArenaView too

ArenaView carried its own copy of the same five transforms. It differs from the
game view in exactly two respects: the arena is a fixed world shown whole, so
the tile size is the tighter of the two axis fits rather than the height fit,
and there is no scrolling, so the left edge is always 0.

That is small enough to absorb into WorldCoordinates as a second named factory.
The raw constructor becomes private and both call sites go through
scrolling(...) or fitToWorld(...), which also reads better than three
positional ints at the call site.

ArenaView is threaded the same way GameWorldView was: paintGL builds one
snapshot, every draw takes a const WorldCoordinates&, and mousePressEvent takes
its own.

ArenaView::widgetToWorld guarded against a near-zero tile size; that guard now
lives in WorldCoordinates as a tilePx fallback to 1.0, alongside the existing
degenerate-world-size fallback, so both factories are total and no conversion
can divide by zero.

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

docs/architecture.md

@@ -358,8 +358,8 @@ Sim and UI run on the same thread for v1. `paintEvent` reads sim state directly
 ### Coordinates and Scrolling
 
 - `GameWorldView` holds a continuous `scrollXTiles` (float), the world X at the *center* of the viewport. A / D input pans this smoothly (REQ-UI-SCROLL) at a position-dependent speed (REQ-UI-SCROLL-SPEED).
-- The world↔widget transform itself lives in `WorldCoordinates` (`lib/core/`), not in the view. It is an immutable value built from the viewport size, the world height, and the scroll center; `tilePx` is derived so the world height fills the viewport (REQ-GW-TILE-SIZE) rather than being fixed. Being a plain value with no Qt Widgets dependency, it is unit-tested (`WorldCoordinatesTest`) even though the widgets around it are not.
-- `GameWorldView::getCoordinates()` builds one per frame in `paintGL` and per event in the mouse handlers, and passes it down: every world-space `draw<X>` takes a `const WorldCoordinates&`, while the screen-space draws (vignette borders, replay overlay, debug text) take none. The snapshot is deliberately never cached in a member — a resize or a scroll would silently invalidate it.
+- The world↔widget transform itself lives in `WorldCoordinates` (`lib/core/`), not in the view. It is an immutable value, built through one of two named factories that differ only in how `tilePx` and the left edge are derived; everything downstream is shared. `scrolling(...)` is the game world: `tilePx` makes the world height fill the viewport (REQ-GW-TILE-SIZE) and the view pans horizontally. `fitToWorld(...)` is the balancing tool's arena: a fixed world shown whole, so `tilePx` is the tighter of the two axis fits and there is no scroll. Being a plain value with no Qt Widgets dependency, it is unit-tested (`WorldCoordinatesTest`) even though the widgets around it are not.
+- `GameWorldView::getCoordinates()` and `ArenaView::getCoordinates()` each build one per frame in `paintGL` and per event in the mouse handlers, and pass it down: every world-space `draw<X>` takes a `const WorldCoordinates&`, while the screen-space draws (vignette borders, replay overlay, debug text) take none. The snapshot is deliberately never cached in a member — a resize or a scroll would silently invalidate it.
 - Conversions are per-call arithmetic rather than a `painter.translate`, because hit-testing needs the inverse (`widgetToWorld` / `widgetToTile`, flooring for a tile) as often as drawing needs the forward direction. Asteroid tiles (`x < 0`) need no special casing — they share the coordinate system with space tiles, which is why the flooring must not be truncation.
 
 ### Culling