Bind Firestore handoff to exact legacy artifact bytes

[Pigbibi]

Summary

• add a versioned live_pool_legacy_exact_bytes Firestore handoff carrying the exact UTF-8 text read from artifacts.live_pool_legacy_path
• fail closed on missing/invalid UTF-8/JSON/non-object bytes, artifact/runtime-identity digest mismatch, or convenience-field mismatch
• cover exact whitespace preservation, one-byte mutation, symbol/map mismatch, and invalid fixture cases offline

Why

The existing Firestore payload reconstructs symbols and symbol_map but does not carry the exact live_pool_legacy.json bytes bound by runtime_evidence_identity.artifacts.live_pool_legacy.sha256.

Validation

• focused release-contract suite: 16 tests passed
• full offline unit suite: 67 tests passed
• Ruff changed/full: passed
• compileall: passed
• package build and installed-package check: passed
• scope/diff/secret checks: passed
• uv lock --check: pre-existing failure reproduced unchanged on clean origin/main; dependency/lock changes are outside this PR

Boundary

Producer half only. No Binance change, provider/cloud call, publish, workflow dispatch/rerun, runtime/deploy/config-sync, credential access, broker/order/capital/paper/shadow/live action, or merge is authorized by this PR.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

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