fix: harden M0 research ledger freshness

[Pigbibi]

Summary

• fail closed on future source timestamps and ready sources that contain errors
• separate fresh current-horizon conflicts from historical stale-horizon drift
• align identifier and v5/v6 provenance constraints across Python and JSON schemas

Safety

• retains research-only and no-order policy
• does not add a selector, runtime, platform, scheduler, worker, credential, or UI integration
• invalid source snapshots are omitted rather than guessed or repaired

Validation

• python -m unittest tests.test_m0_research_ledger passed locally: 9 tests
• real QAR M0 adapter to hardened ledger smoke test passed
• Python compile, JSON syntax, and diff checks passed