test: add exploratory domain mutation testing

.gitignore

@@ -1,4 +1,5 @@
 node_modules/
+.stryker-tmp/
 .jj/
 .codex/
 main.js
@@ -8,4 +9,5 @@ data.json
 .DS_Store
 dist/
 coverage/
+reports/
 tmp/

docs/development.md

@@ -9,6 +9,7 @@ npm ci
 npm run fix
 npm run check
 npm run test:coverage
+npm run test:mutation
 ```
 
 Use the Node.js version in `.node-version`.
@@ -17,6 +18,8 @@ Use `npm run fix` for mechanical cleanup, review its diff, and run `npm run chec
 
 Use `npm run test:coverage` to identify source modules and branches that lack exercised behavior. It reports every authored TypeScript source file, including files not imported by tests, while excluding generated app-server bindings. Open `coverage/index.html` to inspect line-level gaps. Coverage is diagnostic and has no pass/fail threshold; prioritize user-visible behavior and state-transition invariants rather than raising the aggregate percentage.
 
+Use `npm run test:mutation` for an exploratory mutation test of the domain-owned logic under `src/domain/` and `src/features/chat/domain/`. Review surviving mutants individually instead of treating the aggregate score as a quality gate: add tests for meaningful behavior gaps, simplify equivalent or redundant code, and leave mutants alone when neither change improves the durable contract. The run ignores expensive module-initialization mutants, is intentionally manual, and writes its ignored HTML report to `reports/mutation/mutation.html`.
+
 ## Commit Messages
 
 Use [Conventional Commits 1.0.0](https://www.conventionalcommits.org/en/v1.0.0/) for new commits:

package.json

@@ -37,12 +37,15 @@
     "release:prepare": "node scripts/release/prepare.mjs",
     "test": "vitest run --pool threads --maxWorkers 6",
     "test:coverage": "vitest run --pool threads --maxWorkers 6 --coverage",
+    "test:mutation": "stryker run",
     "typecheck": "tsc -p tsconfig.json --noEmit --incremental --tsBuildInfoFile node_modules/.cache/typescript/tsconfig.tsbuildinfo"
   },
   "devDependencies": {
     "@biomejs/biome": "^2.5.3",
     "@commitlint/cli": "^21.2.1",
     "@commitlint/config-conventional": "^21.2.0",
+    "@stryker-mutator/core": "^9.6.1",
+    "@stryker-mutator/vitest-runner": "^9.6.1",
     "@types/node": "^26.1.1",
     "@vitest/coverage-v8": "^4.1.10",
     "concurrently": "^10.0.3",

stryker.config.mjs

@@ -0,0 +1,8 @@
+export default {
+  testRunner: "vitest",
+  coverageAnalysis: "perTest",
+  mutate: ["src/domain/**/*.ts", "src/features/chat/domain/**/*.ts"],
+  ignoreStatic: true,
+  incremental: true,
+  reporters: ["clear-text", "progress", "html"],
+};