fix(codegen): lower bounded series extrema lengths

[luisleo526]

Summary

• Lower a narrowly proven, positive two-choice series length for ta.lowest and ta.highest into two fixed histories advanced on every bar.
• Snapshot the authored selected length once at the call site and fail closed on mutation, ambiguous bindings, incompatible types, builtin shadows, and unbounded input domains.
• Keep arbitrary series lengths, other TA families, persistent targets, UDF-local sites, and request.security evaluators unsupported.

Validation

• Focused finite-choice and derived-length tests: 96 passed.
• Native suite: 1,843 passed, 2 expected skips.
• Pyodide parity gate: 277/277.
• N=3 interaction matrix: exhaustive 8/8, fingerprint 68264ebbde96c96b7193fcb2079759069a755efcd605fb68304fe3dca551c285.
• Frozen 666-source manifest: 661 OK / 5 errors; exactly one delta, Wellman error to OK; the other 665 outputs/errors are byte-identical.
• Canonical Wellman no-cache verification: exact 72/72, 100% coverage, zero count/entry/exit/PnL p90 drift, engine tape a75163f04b2c25ae3f322f48ef27343f2f9f24681712f472ca248b6011138889.
• Forced-no-cache standard sweep: 412/412 classified, 405 OK / 1 no-trades / 5 transpile / 1 known run error, 0 unresolved timeouts, 0 tier downgrades.
• Pinned engine corpus grade: 247 Excellent / 4 Strong / 1 documented anomaly.