Validate fixture identifier relationships

[rogerchappel]

Summary

• reject duplicate vendor, invoice, and payment identifiers with deterministic row/field diagnostics
• require invoice vendors and payment invoices/vendors to resolve, including exact payment-to-invoice vendor agreement
• document the relationship and duplicate-ID contract for custom fixtures

Commits

• a28a303 Validate fixture identifier relationships
• 62b8689 Document fixture relationship requirements

Verification

• npm run release:check (39 tests; syntax, CLI smoke, documentation install smoke, package dry-run)
• git diff --check origin/main...HEAD
• author and committer identity verified for every proposed commit as Roger Chappel <miscanalysis@gmail.com>

Scope

3 files changed, 117 insertions, 3 deletions.

[rogerchappel]

Automated merge note

Triage class: auto-merge

Summary: Validates fixture identifier relationships, rejects duplicate identifiers and inconsistent invoice/payment references, adds regression coverage, and documents the fixture contract.

Checks run: CI — Repository hygiene: SUCCESS; worker verification: npm run release:check (39 tests, syntax, CLI smoke, documentation install smoke, package dry-run); git diff --check origin/main...HEAD.

Rebased/CI-repaired: No.

Verified head SHA: 62b8689eee91d9e17de5ea13c5c12ecd4385b92c.