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/auth/vitest.config.ts

@@ -1,7 +1,18 @@
 import path from "node:path";
 import { mergeConfig } from "vitest/config";
 import { nodeVitestConfig } from "@repo/core-typescript/vitest.base.node";
+import {
+  DEFAULT_COVERAGE_BANDS,
+  vitestThresholdsFromBands,
+} from "@repo/core-shared/conformance/coverage";
 
+// Coverage thresholds derived from DEFAULT_COVERAGE_BANDS via the shared
+// helper — one source of truth for the conventional band shape across all
+// features (ADR-020). The feature.manifest.ts `coverage.bands` section also
+// declares these for boot-time `assertFeatureConformance` (which reads the
+// manifest directly, not the vitest config). For features that need
+// non-default bands, override here AND in the manifest, then add a drift
+// test in core-shared/conformance/.
 export default mergeConfig(nodeVitestConfig, {
   test: {
     coverage: {
@@ -18,30 +29,7 @@ export default mergeConfig(nodeVitestConfig, {
         // React Query option builders — integration-tested in apps
         "src/ui/**",
       ],
-      thresholds: {
-        "src/entities/**": {
-          statements: 100,
-          branches: 100,
-          functions: 100,
-          lines: 100,
-        },
-        "src/application/use-cases/**": {
-          statements: 100,
-          branches: 95,
-          functions: 100,
-          lines: 100,
-        },
-        "src/interface-adapters/controllers/**": {
-          statements: 100,
-          branches: 95,
-          functions: 100,
-          lines: 100,
-        },
-        statements: 80,
-        branches: 75,
-        functions: 80,
-        lines: 80,
-      },
+      thresholds: vitestThresholdsFromBands(DEFAULT_COVERAGE_BANDS),
     },
   },
   resolve: {