Validate custom fixture schemas before analysis

[rogerchappel]

Summary

• validate fixture collection shapes and every detector/scenario input field before analysis
• reject malformed or impossible calendar dates with stable, row-specific INVALID_FIXTURE_SCHEMA diagnostics
• constrain payment methods, preserve amount diagnostics, and document the custom-fixture contract

Commits

• a320fe4 fix: validate fixture schema before analysis
• 4ba52a6 test: cover invalid fixture shapes and dates
• 1034391 docs: define custom fixture schema

Verification

• npm run release:check (31 tests, syntax checks, CLI smoke, documentation install smoke, package smoke)
• git diff --check origin/main...HEAD
• author and committer identity verified for every commit as Roger Chappel <miscanalysis@gmail.com>

Existing open PRs #7 and #10 only update GitHub Actions and do not overlap this fixture-validation change.

[rogerchappel]

Automated merge note

Triage class: auto-merge

Summary: Validates custom fixture schemas before analysis, with strict date/schema diagnostics, regression tests, and documentation (7 files, +139/-4); final diff re-read found routine code, tests, and documentation only.

Checks run: GitHub required/reported check Repository hygiene completed SUCCESS; live mergeability=MERGEABLE; branch protection requires one review only and exposes no required status-check contexts; final diff, protection, and head SHA reverified immediately before merge.

Rebased/CI-repaired: No.

Verified head SHA: 10343919124cc27ce860bad5e53765c19eaff88e.