Validate per-scenario fixture minimums

[rogerchappel]

Summary

• add stable scenario-specific fixture requirement errors
• cover empty invoice and vendor seed collections
• document which scenarios require existing entities

Commits

• test: cover empty scenario seed collections
• fix: validate scenario fixture minimums
• docs: define scenario fixture requirements

Verification

• npm ci
• npm run release:check (33 tests, syntax checks, CLI smoke, documentation install smoke, package dry run)
• git diff --check origin/main...HEAD

All commit authors and committers verified as Roger Chappel miscanalysis@gmail.com.

[rogerchappel]

Automated merge note

Triage class: auto-merge

Summary: Validates minimum fixture seed entities for affected scenarios, with regression tests and README guidance.

Checks run: Repository hygiene (CI) SUCCESS; npm ci, npm run release:check, and git diff --check passed before review.

Rebased/CI-repaired: No.

Verified head SHA: f6b96b2e53ed9b66fa4087b73c52c262f510ed4a.

Final review: self-authored, non-draft, mergeable; 3 files and 45 changed lines; only review protection remains.