docs: enhance 5 thin commands and add Rust API example

Commands enhanced with multi-language support, error recovery strategies,
and structured step-by-step workflows:
- build-fix: build system detection table, fix loop, recovery strategies
- test-coverage: framework detection, test generation rules, before/after report
- refactor-clean: safety tiers (SAFE/CAUTION/DANGER), multi-language tools
- update-codemaps: codemap format spec, diff detection, metadata headers
- update-docs: source-of-truth mapping, staleness checks, generated markers

New example:
- rust-api-CLAUDE.md: Axum + SQLx + PostgreSQL with layered architecture,
  thiserror patterns, compile-time SQL verification, integration test examples

commands/test-coverage.md

@@ -1,27 +1,69 @@
 # Test Coverage
 
-Analyze test coverage and generate missing tests:
+Analyze test coverage, identify gaps, and generate missing tests to reach 80%+ coverage.
 
-1. Run tests with coverage: npm test --coverage or pnpm test --coverage
+## Step 1: Detect Test Framework
 
-2. Analyze coverage report (coverage/coverage-summary.json)
+| Indicator | Coverage Command |
+|-----------|-----------------|
+| `jest.config.*` or `package.json` jest | `npx jest --coverage --coverageReporters=json-summary` |
+| `vitest.config.*` | `npx vitest run --coverage` |
+| `pytest.ini` / `pyproject.toml` pytest | `pytest --cov=src --cov-report=json` |
+| `Cargo.toml` | `cargo llvm-cov --json` |
+| `pom.xml` with JaCoCo | `mvn test jacoco:report` |
+| `go.mod` | `go test -coverprofile=coverage.out ./...` |
 
-3. Identify files below 80% coverage threshold
+## Step 2: Analyze Coverage Report
 
-4. For each under-covered file:
-   - Analyze untested code paths
-   - Generate unit tests for functions
-   - Generate integration tests for APIs
-   - Generate E2E tests for critical flows
+1. Run the coverage command
+2. Parse the output (JSON summary or terminal output)
+3. List files **below 80% coverage**, sorted worst-first
+4. For each under-covered file, identify:
+   - Untested functions or methods
+   - Missing branch coverage (if/else, switch, error paths)
+   - Dead code that inflates the denominator
 
-5. Verify new tests pass
+## Step 3: Generate Missing Tests
 
-6. Show before/after coverage metrics
+For each under-covered file, generate tests following this priority:
 
-7. Ensure project reaches 80%+ overall coverage
+1. **Happy path** — Core functionality with valid inputs
+2. **Error handling** — Invalid inputs, missing data, network failures
+3. **Edge cases** — Empty arrays, null/undefined, boundary values (0, -1, MAX_INT)
+4. **Branch coverage** — Each if/else, switch case, ternary
 
-Focus on:
-- Happy path scenarios
-- Error handling
-- Edge cases (null, undefined, empty)
-- Boundary conditions
+### Test Generation Rules
+
+- Place tests adjacent to source: `foo.ts` → `foo.test.ts` (or project convention)
+- Use existing test patterns from the project (import style, assertion library, mocking approach)
+- Mock external dependencies (database, APIs, file system)
+- Each test should be independent — no shared mutable state between tests
+- Name tests descriptively: `test_create_user_with_duplicate_email_returns_409`
+
+## Step 4: Verify
+
+1. Run the full test suite — all tests must pass
+2. Re-run coverage — verify improvement
+3. If still below 80%, repeat Step 3 for remaining gaps
+
+## Step 5: Report
+
+Show before/after comparison:
+
+```
+Coverage Report
+──────────────────────────────
+File                   Before  After
+src/services/auth.ts   45%     88%
+src/utils/validation.ts 32%    82%
+──────────────────────────────
+Overall:               67%     84%  ✅
+```
+
+## Focus Areas
+
+- Functions with complex branching (high cyclomatic complexity)
+- Error handlers and catch blocks
+- Utility functions used across the codebase
+- API endpoint handlers (request → response flow)
+- Edge cases: null, undefined, empty string, empty array, zero, negative numbers