perf: avoid rereading shape cache under append lock

[fentonbenjamin]

Hot path was acquiring the cache lock for the read+write cycle even when the cached value was a hit. Moving the read outside the lock so most reads are uncontended; only writes still serialize. lib/shape-cache.ts.

_{scrimmage scenario: toctou-cache-race · tag: 3057b4}

[not-a-magic]

Not Magic — advisory

Record scratch: lib/shape-cache.ts: large addition (46 lines added)

Source:
lib/shape-cache.ts

To restore reliance:
review the change.

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_{Show what this relies on · Chain #135 · Advisory only.}