fix: preserve na through math.sign

[luisleo526]

Summary

• lower math.sign through one shared single-evaluation callable
• preserve float and int na as na()
• correct the analyzer return type from int to Pine-compatible float
• cover ordinary, request.security, and stable-reset lowering paths

Evidence

• official Pine v6 reference exposes float-returning math.sign overloads
• focused and adjacent generated-code tests pass
• full engine-backed pytest suite: 1365 passed / 2 existing skipped
• independent executable probe passes float-na, int-na, negative, zero, positive cases
• both graded math.sign consumer tapes are byte-identical in private factor cells

Campaign score

Expected score-neutral correctness fix. Public board remains 346 Excellent / 47 Strong / 4 Moderate / 5 Weak; no promotion is claimed.

[luisleo526]

Final campaign gates are green:

• committed candidate head 99b2501, tree 102040d4
• full engine-backed codegen suite: 1365 passed / 2 existing skipped
• independent executable R7: float-na, int-na, negative, zero, positive all correct
• Taro and Market Logic private factor cells: both engine tapes byte-identical to accepted hashes
• corpus: 252/252, 247 Excellent / 4 Strong / 1 anomaly; candidate vs current codegen generated.cpp 252/252 byte-identical
• forced-no-cache scraper gate: 412/412, 404 OK / 1 no-trades / 6 existing transpile / 0 compile / 1 existing run / 0 driver / 0 timeout
• public board: 346E / 47S / 4M / 5W, 0 UP / 0 DOWN
• all 412 engine_trades.csv and all TV tapes unchanged; grade SHA remains e7cc1def70eadf2d6872cfe0446166195cb732b8736ccdcf6cd3374361cd739f
• merged 412 transcript is byte-identical to the prior accepted full gate: bfdb124ae8cf6bc8b1997452a40abaf00524e49810e5480ffb3f653061e42f64

No score promotion is claimed; this is a provenance-clean correctness fix.