[IR-P0][WS6] Golden vectors format and CI harness skeleton

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Problem statement

Without shared fixtures and harness structure, Phase 1+ changes cannot be validated consistently.

Intended invariant

Golden vectors provide deterministic IR conformance fixtures; CI harness can execute and report conformance status.

Scope boundaries

• Fixture format and harness skeleton only
• No full runtime dual-run implementation

Deliverable checklist

• Define golden vector schema
• Add representative sample vectors
• Add CI harness skeleton and reporting shape
• Document how future phases extend vectors

Validation plan

CI job runs harness skeleton and validates sample vectors.

Rollback/failure mode

If harness shape is contested, keep vectors merged and isolate harness wiring in a follow-up issue.

Roadmap reference

Roadmap reference: docs/plans/2026-06-19-001-ir-first-roadmap.md (Phase 0)

Migration impact

none - Test/infra harness groundwork only; no direct user-facing behavior changes.

Documentation impact

update-existing - Update roadmap and migration guide with harness coverage and extension instructions.

Sync status

Synced with roadmap: 2026-06-19

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Roadmap sync update (2026-06-19): IR work branch policy is now explicit -- implementation branches must be created from feat/ir-first and PRs must target feat/ir-first. main promotion is deferred until the full IR program is complete and release-ready.

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Phase 0 artifact update (branch feat/ir-first):

• Golden vector format doc: tests/fixtures/ir_vectors/README.md
• Seed vectors: tests/fixtures/ir_vectors/*.json
• CI harness skeleton: tests/test_ir_vectors_contract.py
• CI wiring: .github/workflows/ci.yml (explicit IR vector harness step).
  Validation run: uv run pytest -v tests/test_ir_vectors_contract.py (pass).
  Roadmap sync updated in docs/plans/2026-06-19-001-ir-first-roadmap.md in same work session.

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Completed in PR #75 (merged): #75

Evidence:

• Vector format + fixtures: tests/fixtures/ir_vectors/README.md, tests/fixtures/ir_vectors/*.json
• CI harness skeleton: tests/test_ir_vectors_contract.py
• CI trigger for feat/ir-first PRs in .github/workflows/ci.yml