refactor(iso): drop the unused stageSize from the overlay fit input (#741)

Since #713 the overlay fit envelope reserves no nudge budget, and since
#725 _isoScene passes `stageSize: null` while resolveIsoOverlayFitEnvelope
never reads the field. The room focus still built a { width, height }
object from the stage for nothing. The optional field is removed from
IsoOverlayFitEnvelopeInput together with both call-site arguments.

The #725 AC3 unit compared bounds with stageSize null and 1000x500, which
is now meaningless; it checks instead that the fit bounds follow only
scene.frame and the tiles: the same bounds for every stage aspect, a moved
frame moves them, an enclosing frame is returned as is.

Issue: #741
User-Visible: no
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018qZfe7YS4rqEMKoVeS3GKd

src/iso-scene-render.ts

@@ -648,7 +648,6 @@ export function isoOverlaySceneBounds(
 export interface IsoOverlayFitEnvelopeInput {
   baseBounds: Rect;
   entries: readonly IsoOverlayRenderEntry[];
-  stageSize?: { width: number; height: number } | null;
   targetView(bounds: Rect): Rect | null;
   ownerId?: string;
 }