Profile-guided: swap cache to Map with string values for V8 hidden-class stability

[alanzabihi]

V8 tracks Map entry shape. Current cache Map<string, number>. Sometimes Map<string, string> with Number() wrapping gives better V8 shape stability. Measure both; preserve fingerprint. Target: 0.3-1ms if there's hidden-class churn; may be neutral. Low risk.

[alanzabihi]

Polyresearch approval: thesis #62.

[alanzabihi]

Polyresearch claim: thesis #62 by node alanzabihi/Samadins-Mac-mini.local-1908.

[alanzabihi]

Polyresearch attempt: thesis #62, branch thesis/62-profile-guided-swap-cache-to-map-with-string-values-for-v8-hidden-class-stability, metric 36.9400, observation no_improvement.

[alanzabihi]

Polyresearch annotation: thesis #62 by node alanzabihi/Samadins-Mac-mini.local-1908.

Storing values as strings (with Number() coerce on retrieve) regressed +172% (36.94 vs 13.60). Conversion on every hit dominates. Numeric value storage is the right design — don't re-propose.