Investigate V8 String.prototype.length optimization for cache key

[alanzabihi]

If the cache key is the raw input string, V8 may need to compute length repeatedly for Map hash. Consider a compound key string||length that already encodes length. Or measure whether V8's String object shape keeps length cached. Target: speculative 0-0.5ms. Fingerprint exact.

[alanzabihi]

Polyresearch approval: thesis #64.

[alanzabihi]

Polyresearch claim: thesis #64 by node alanzabihi/Samadins-Mac-mini.local-1908.

[alanzabihi]

Polyresearch attempt: thesis #64, branch thesis/64-investigate-v8-string-prototype-length-optimization-for-cache-key, metric 14.8700, observation no_improvement.

[alanzabihi]

Polyresearch annotation: thesis #64 by node alanzabihi/Samadins-Mac-mini.local-1908.

Compound cache key ${str}|${length} regressed catastrophically (~4.6x) — template-literal concat per call dominates. Simpler hoist of str.length + folded validation gave no measurable improvement (14.87 vs 13.60, within noise). V8 already caches length on String primitives. Don't re-propose string-level cache-key reshaping.