feat(core-shared/conformance): manifest coverage schema + vitest helper

First implementation milestone of the agent-first coverage architecture
(ADR-020, PRD 2026-05-13). Lands the keystone — coverage bands as a
typed declaration in feature.manifest.ts plus a helper that derives
vitest threshold shapes from them.

New file packages/core-shared/src/conformance/coverage.ts (self-
contained, no relative imports — loadable at vitest config time):
- CoverageBand / CoverageBands / CoverageManifest / VitestThresholds
  types
- DEFAULT_COVERAGE_BANDS (baseline 80/75/80/80; entities 100/100/100/
  100; use-cases + controllers 100/95/100/100) — matches ADR-011
- DEFAULT_MUTATION_SCORE (80) + DEFAULT_MUTATION_TARGETS (entities +
  use-cases)
- getCoverageBands / getMutationConfig — manifest -> resolved bands,
  with default fallback for missing layers
- vitestThresholdsFromBands / vitestThresholdsFromManifest — convert
  to vitest's coverage.thresholds shape with the layer-to-glob
  mapping

define-feature.ts gains the optional coverage field on FeatureManifest
(imports its type from coverage.ts to avoid a relative-import cycle
at config-load time).

Exposed via two subpaths: @repo/core-shared/conformance (re-exports
for source/test code) and @repo/core-shared/conformance/coverage
(direct subpath safe to load from vitest configs, bypasses the index
re-export chain that Node ESM doesn't auto-extension-resolve).

Auth wired as proof-of-concept:
- packages/auth/src/feature.manifest.ts declares its coverage section
- packages/auth/vitest.config.ts imports the helper +
  DEFAULT_COVERAGE_BANDS and emits thresholds via
  vitestThresholdsFromBands(DEFAULT_COVERAGE_BANDS) — no more
  hand-maintained per-glob thresholds block.

Verified: 175/175 tests pass; 14/14 typechecks clean; auth coverage
green (21 tests, 93.77% overall, all per-layer 100% bands hold).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core-shared/src/conformance/index.ts

@@ -1,6 +1,21 @@
 export type { Instrumented, Captured } from "./brands";
 export type { FeatureManifest, UseCaseManifest } from "./define-feature";
 export { defineFeature } from "./define-feature";
+export type {
+  CoverageBand,
+  CoverageBands,
+  CoverageManifest,
+  VitestThresholds,
+} from "./coverage";
+export {
+  DEFAULT_COVERAGE_BANDS,
+  DEFAULT_MUTATION_SCORE,
+  DEFAULT_MUTATION_TARGETS,
+  getCoverageBands,
+  getMutationConfig,
+  vitestThresholdsFromBands,
+  vitestThresholdsFromManifest,
+} from "./coverage";
 export type { ProductionUseCase } from "./production-use-case";
 export {
   attachBrand,