[Bug]: [P3] Thief cosmetic can play fully off-screen

Imported from the engineering backlog (_bmad-output/implementation-artifacts/backlog.md, story event-visuals).
Priority P3 (do-first order) | Severity: low (cosmetic) | Status: open | Epic: — | Type: review-deferral | Recorded: 2026-07-06

What happened?

Ready. Thief cosmetic can play fully off-screen. TowerEngine.renderThief anchors the thief's Y to a random tenanted floor (worldToScreenY(thiefFloor)); in a tall tower that floor is often outside the viewport, so the run animates invisibly (regression from the old always-visible viewHeight*0.66). Accepted tradeoff of grounding the thief on a real floor + the engine/render separation (the engine can't know the camera), and the player still gets the log/toast line. If it reads as "missing the cosmetic," bias selection toward the lobby / a low floor (like the VIP limo), or nudge the camera to the thief's floor for the run. (gds-code-review, thief-grounding: Blind + Edge Case Hunter.)

Start at TowerEngine.renderThief and trace how thiefFloor feeds worldToScreenY; compare that path with the former viewHeight*0.66 anchor. Check the event-visuals story and VIP limo/lobby selection context, then confirm the chosen visibility behavior in a tall tower. Done means the thief cosmetic is not allowed to animate entirely outside the viewport.