perf(region): memoize rebuildBlockLedger per snapshot to cut O(B^2*N) rebuilds (#109)

[ranxianglei]

What

Fixes #109 — rebuildBlockLedger was being recomputed many times inside hot loops instead of once per tool call.

Root cause

rebuildBlockLedger scans the full durable log and calls summarySeqOfCompaction (itself O(N)) once per compaction block → O(B·N) per call. Several helpers wrap it (blockRegistry, expandShadowedSeqs, blockRefForSummarySeq, compactionIdsOfKernelBlocks, summarySeqOfKernelBlock) and are invoked per element in loops:

• search_context → buildSearchDocs loops expandShadowedSeqs over every block → O(B²·N)
• compress → handleCompress calls blockRefForSummarySeq×2 + compactionIdsOfKernelBlocks per range → O(R·B·N)
• tier-nudge → summarySeqOfKernelBlock per target block

So search / compress / nudge latency grew super-linearly with the number of blocks.

Fix

Memoize rebuildBlockLedger on the input snapshot array via a module-level WeakMap, guarded by array length. Safe because:

• within one tool invocation every caller passes the same stable snapshot (sessionEventsOf reuses it until the next append);
• the log is append-only, so same reference + same length ⇒ identical content;
• after an append the snapshot is either a new array (copy-on-write) or grows in length → the key misses → recompute.

The function is pure and no caller mutates the returned ledger, so sharing the cached array is correct. Net effect: each tool call rebuilds the ledger once (O(B·N)) instead of per element — a ~B× reduction on the hot paths. Zero signature changes; all callers benefit automatically.

Verification

• npm run typecheck ✅
• npm test → 209/209 pass (added an idempotency regression test asserting repeated calls on one snapshot return an identical ledger)
• npm run build ✅ — dist/ regenerated for CI's dist-drift check; dist/region.d.ts is byte-identical to main (no public type change)

Files

• src/region.ts (+7): the WeakMap memo + length guard
• tests/region.test.ts (+21): idempotency regression test
• dist/index.js, dist/index.js.map: rebuilt bundle

[ranxianglei]

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