test: add coverage reporting with regression thresholds

Add v8 coverage via a test:coverage script and vitest config: measure src
only, exclude the types-only deps.ts and the bin entrypoint main.ts, and
gate on ratcheted thresholds (statements 85, branches 78, functions 90,
lines 85) set just under current coverage. CI runs the unit+integration
tier through coverage so a real drop fails the build.

package.json

@@ -19,6 +19,7 @@
     "typecheck": "tsc --noEmit",
     "test": "vitest run",
     "test:watch": "vitest",
+    "test:coverage": "vitest run --coverage",
     "test:e2e": "vitest run --config vitest.e2e.config.ts"
   },
   "dependencies": {
@@ -28,6 +29,7 @@
   },
   "devDependencies": {
     "@types/node": "^20.19.0",
+    "@vitest/coverage-v8": "^3.2.7",
     "typescript": "^5.8.0",
     "vitest": "^3.2.0"
   }