fix: validate fixture timing invariants

[rogerchappel]

Summary

• reject non-increasing fixture timestamps with file and sample diagnostics
• validate multi-sample frame duration, total duration, and baseline ranges
• cover normalization and CLI failure paths and document the fixture contract

Commits

• 56d1b77 test: cover invalid fixture timing
• e233734 fix: enforce fixture timing invariants
• dd19d96 docs: define fixture timing contract

Verification

• npm run release:check (45 tests, typecheck, build, smoke, installed-package/import checks, package dry-run)
• git diff --check origin/main...HEAD
• author and committer identity verified for every commit

[rogerchappel]

Automated merge note

Triage class: auto-merge

Summary: Validates fixture timing invariants, with focused CLI and normalization regression coverage plus fixture-contract documentation.

Checks run: GitHub CI verify SUCCESS; isolated-worktree npm ci and npm run release:check SUCCESS (45 tests, syntax, typecheck, build, smoke, installed-package/import and package dry-run checks); git diff --check origin/main...HEAD SUCCESS; final live diff, commit identities, mergeability, branch protection, and head SHA reverified.

Rebased/CI-repaired: No.

Verified head SHA: dd19d96a76851c64082b9cd334e9653ae513c00b.