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/package.json

@@ -7,6 +7,7 @@
     ".": "./src/index.ts",
     "./audit": "./src/audit/index.ts",
     "./conformance": "./src/conformance/index.ts",
+    "./conformance/coverage": "./src/conformance/coverage.ts",
     "./di": "./src/di/index.ts",
     "./di/bind-protocols": "./src/di/bind-protocols.ts",
     "./di/bind-context": "./src/di/bind-context.ts",
@@ -52,8 +53,12 @@
     "@sentry/react": "^10.51.0"
   },
   "peerDependenciesMeta": {
-    "@sentry/node": { "optional": true },
-    "@sentry/react": { "optional": true }
+    "@sentry/node": {
+      "optional": true
+    },
+    "@sentry/react": {
+      "optional": true
+    }
   },
   "devDependencies": {
     "@opentelemetry/context-async-hooks": "^1.28.0",