story-kit: create 69_story_test_coverage_qa_gate

.story_kit/work/1_upcoming/69_story_test_coverage_qa_gate.md

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+---
+name: Test Coverage QA Gate
+test_plan: pending
+---
+
+# Story 69: Test Coverage QA Gate
+
+## User Story
+
+As a developer, I want a `script/test_coverage` script that collects and reports test coverage for both Rust and frontend code, so that it can be run as a QA gate when a work item enters `3_qa/`.
+
+## Acceptance Criteria
+
+- [ ] `script/test_coverage` exists and is executable
+- [ ] It runs Rust tests with coverage collection (e.g. `cargo llvm-cov` or `cargo tarpaulin`) and reports line coverage percentage
+- [ ] It runs frontend tests with coverage collection (e.g. `vitest --coverage`) and reports line coverage percentage
+- [ ] The script exits non-zero if coverage is below a configurable threshold (default: the current baseline, so coverage can only go up)
+- [ ] The QA stage (`3_qa/`) in the server pipeline runs `script/test_coverage` as a gate check
+- [ ] Work items cannot progress from `3_qa/` to `4_merge/` if the coverage gate fails
+
+## Out of Scope
+
+- Per-file or per-function coverage reporting
+- Coverage badges or dashboards
+- Branch coverage (line coverage is sufficient for now)