Stop the map holding a 200 px level over the sharp one

[ryanbarlow97]

The movement map (and any TileLayer) could open blurry and stay that way until the next zoom. #115 didn't fix it because it wasn't the animation.

Cause. When the tile manifest is already cached (coming back to the Movement tab, for example), TileLayer renders before its viewport is measured. It sees scale 0, picks level 0 (200 px) and records it as settledLevel. Once the viewport is measured, the whole-map view wants level 2. Level 2 is also the backdrop, so there are no "current" tiles, and currentShown can never become true. The settled level was therefore never updated, and the 200 px level stayed held on top of the 800 px backdrop.

Fix.

• At the backdrop's level, the level settles once the backdrop has loaded.
• A held level that is coarser than the backdrop stands down as soon as the backdrop has loaded.

Checks.

• New test: render at scale 0, then at the whole-map scale. Without the fix, level 0 stays visible.
• Updated test: zooming out to an already-loaded backdrop now drops the sharper held tiles straight away rather than holding them indefinitely.
• vitest run: 1971 passed. tsc --noEmit is clean.
• Headless Chromium against the live tiles, opening Movement, then Players, then Movement. Before the fix, a level-0 tile was visible over the 16 level-2 tiles, matching the report. After the fix, only level 2 is visible.

🤖 Generated with Claude Code

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Reviewing files that changed from the base of the PR and between a687876 and dd2733e.

📒 Files selected for processing (2)

• frontend/app/components/map/TileLayer.test.tsx
• frontend/app/components/map/TileLayer.tsx

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