Docs: Unit coverage baseline and priorities

[mjkatgithub]

Planning

• Milestone: Phase 4
• Priority: P2
• Size: S
• Estimate: 2 SP
• Epic: Epic: Raise unit coverage toward critical-path targets #129

Goal

Publish a coverage baseline so threshold raises and module targets are
data-driven, not guessed.

Scope

• Add docs/testing-coverage.md with current npm run test:coverage summary
• Table: folder → lines/functions/branches % → priority (P1/P2/P3)
• List top 10 uncovered critical files (Matrix, timeline, auth)
• Note repo-wide ~49% vs module targets (~90% on critical paths only)

Branch

feature/coverage-baseline-doc

Acceptance Criteria

• Doc committed with reproducible command to refresh numbers
• Linked from README "Test layers"
• Epic Epic: Raise unit coverage toward critical-path targets #129 scope checkbox for baseline report can be ticked

Test Checklist

• Regenerate coverage and verify doc matches within ±2%

Notes

No threshold change in this issue — documentation only.