Enforce strict AI signal ingestion and advisory v6

[Pigbibi]

Summary

• strictly validate RSCP v1/v2 AI signal inputs, including model/scoring version for v2
• make malformed, future, stale, expired, or authority-bearing inputs deterministic no-ops
• prevent self-reported AI confidence from increasing recommendation scores
• bind parsing and input_digest to the same frozen bytes
• emit advisory v6 by default while retaining v5 read/preview compatibility
• default scheduled periods to completed UTC dates/windows

Safety boundary

• reject order, allocation, target-weight, and live-authority fields
• sanitize failures and preserve advisory-only behavior
• no broker, runtime, provider, or deployment activation

Validation

• PYTHONPATH=src python3 -m pytest -q — 349 passed
• changed-file Ruff — passed
• compileall — passed
• four changed workflows actionlint — passed
• git diff --check — passed
• independent specification and quality review — passed