feat(test): WP-46 — Vitest coverage (report-only)

Enable coverage via the @angular/build:unit-test builder's first-class options: a
`coverage` configuration on the test target (v8 provider, text-summary/html/lcov,
excludes for specs/stories/contracts/generated client) + `npm run test:coverage`.
Report-only (no thresholds) — visibility first. CI's test step now runs test:coverage
(strict superset: fails on a failing test, never on coverage). Baseline ~71% stmts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/project/backlog/README.md

@@ -90,7 +90,7 @@ for its existing violations, so every WP ends green.
 | [WP-43](WP-43-scaffold-generators.md)          | Runnable generators: value-object / form-machine / bff-endpoint / ui-component    | 8 · platform/DX/showcase    | todo   |
 | [WP-44](WP-44-context-generator.md)            | Runnable generator: `gen:context`                                                 | 8 · platform/DX/showcase    | todo   |
 | [WP-45](WP-45-create-ssp-generator.md)         | `create-ssp` bootstrap generator (mechanise new-ssp)                              | 8 · platform/DX/showcase    | todo   |
-| [WP-46](WP-46-vitest-coverage.md)              | Vitest coverage (report + report-only thresholds)                                 | 8 · platform/DX/showcase    | todo   |
+| [WP-46](WP-46-vitest-coverage.md)              | Vitest coverage (report + report-only thresholds)                                 | 8 · platform/DX/showcase    | done   |
 
 Sequencing dependencies (stated in the WPs too): 01 before 10–15 (axe covers story churn);
 03/04 before 05–09 (boundaries stop new violations during refactors); 06 before 07 (typed