feat(undo): byte budget, meta-only eviction, compaction (PR11 part 1)

[barakbl]

First of two parts for PR11 (bounding the tile-undo delta chain). This part replaces PR10's eager fold-into-base with the plan's deferred foldedIds model + byte budget + compaction. Part 2 adds QuotaExceededError recovery + the one-time toast. On-mode only; flag off is unchanged.

What's in it

• Meta-only eviction - TileShadow moves the oldest active entries into a folded list (no pixel work) while the window exceeds MAX_UNDO or the byte budget = min(32MB, storage.estimate().quota / 20) (sized at boot via applyBudget). Keeps ≥1 active entry, so a user at the stack bottom overshoots transiently rather than losing the entry they just made. After a commit the pointer is at the tip, so eviction only folds entries strictly below it (pointer remaps as it folds).

• Compaction - once folded passes 30, one enqueued step bakes every folded entry into the base and bumps the base version; the store rewrites the base and deletes the folded rows in the same tx (never folded rows without a base).

• Reconstruction / serialize / hydrate apply base + folded + active, so boot replays folded rows and stays exact across a compaction boundary.

• Store - meta2 gains foldedIds; folded + active share one entry:<id> namespace tracked by savedRowIds. A pre-PR11 meta2 (no foldedIds) still loads with an empty folded list, so on-mode early sessions don't drop to the v1 ladder.

Acceptance

• fold / pointer remap, undo-redo-after-eviction - unit tests reconstruct exactly within the active window after eviction.
• byte-budget eviction - a 1-byte budget folds down to the kept entry; a 4KB-budget 500-stroke soak stays under budget.
• compaction state-equivalence - exact reconstruction across the 30-fold boundary; a store test asserts the base rewrite + folded-row delete land in one tx.
• 500-stroke soak stays under budget, folded ≤ 30 (boot-replay cap), reconstructs exactly.
• Live tiled-boot.mjs gains an eviction phase (folded rows round-trip through IDB, boot == live paint): 10/10 on real GPU Chrome.

Checks

npm run lint clean, npx tsc --noEmit clean, npx vitest run 825/825, npm run build:app OK. Live: tiled-boot.mjs 10/10, tiled-restore.mjs 8/8 (no regression).