Story 27: Coverage tracking (full-stack)

Add end-to-end coverage tracking: backend collects vitest coverage,
records metrics with threshold/baseline tracking, and blocks acceptance
on regression. Frontend displays coverage in gate/review panels with
a "Collect Coverage" button. Includes 20 Rust tests, 17 Vitest tests,
and 14 Playwright E2E tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.story_kit/stories/archived/27_protect_tests_and_coverage.md

@@ -0,0 +1,37 @@
+---
+name: Coverage Tracking
+test_plan: approved
+---
+
+# Story 27: Coverage Tracking
+
+## User Story
+As a user, I want the workflow to track test coverage and block acceptance when coverage regresses, so quality guardrails cannot be weakened silently.
+
+## Acceptance Criteria
+- [x] The workflow fails if coverage drops below the defined threshold.
+- [x] Coverage regression is reported clearly before acceptance.
+
+## Test Plan (Approved)
+
+### Backend (Rust) — Unit
+
+**AC1: Workflow fails if coverage drops below threshold**
+- `workflow::check_coverage_threshold()` fails when coverage % < configured threshold
+- Passes when coverage >= threshold
+
+**AC2: Coverage regression reported clearly before acceptance**
+- `workflow::evaluate_acceptance_with_coverage()` includes coverage delta when coverage dropped
+- `AcceptanceDecision` extended with `coverage_report` field
+- Acceptance blocked with clear message when baseline exists and current < baseline
+
+### Frontend (Vitest + Playwright)
+
+**AC1:** Gate panel shows "Coverage below threshold (X% < Y%)" and coverage display
+**AC2:** Review/gate panels display coverage regression text and summary
+**E2E:** Blocked acceptance displays coverage reasons; green coverage when above threshold
+
+## Out of Scope
+- Introducing new test frameworks beyond those listed in `specs/tech/STACK.md`.
+- Large refactors solely to improve coverage.
+- Path-based test file detection (not reliable for languages with inline tests like Rust).