Add provider-neutral semantic probes and independent validation calls

[thomaswillner]

What changed

• adds provider-neutral semantic evidence models and deterministic recorded fixtures
• adds errata semantic-test with exact verified/failed/unknown exits
• binds qualifying G2 review evidence to a live reviewed commit and comprehensive canonical surface digest
• publishes independent review, implementation, and Phase 3 system-nomination calls
• records evidence-bounded publication strategy and keeps readiness claims synchronized

Evidence boundary

This is experimental work in progress. Overall verdict remains NOT_PROD_READY; G2 remains BLOCKED. Phase 2 implementation and the internal adapter-conformance harness are complete, including durable quarantine, complete vector coverage, exact corpus/source/binding identity, and executable anti-vacuity controls. No internal agent review, local test, CI run, invitation, or publication call is represented as independent external evidence.

Validation

• independent exact-head integration reviews at 9eba33f: SPEC PASS and STANDARDS PASS
• make check: 358 tests plus repository, claim, readiness, publication, and partial-coverage demo gates
• exact-head CI: Python 3.11 and 3.13 PASS — https://github.com/thomaswillner/llm-errata/actions/runs/31753295128
• make links: 53 real citations resolved or were client-blocked
• git diff --check and branch-range diff check: clean
• secret-safe content scan: no credential/private-key patterns

Dependency

Targets agent/prod-readiness because PR #3 must receive separate CODEOWNER approval and merge first. This PR remains NOT_PROD_READY; merge advances internal publication work only and does not satisfy external G2-G6 evidence.

Related calls: #4, #5, #6.