test: add vitest, coverage and mutation gate with availability domain core

Pure logic lives in apps/web/src/domain and must stay at 100% coverage and a
100% mutation score. The first unit is the availability logic for the hero:
parsing, resolving and formatting the next free capacity block.

Vitest is pinned to 4.1.11 because @stryker-mutator/vitest-runner 10.0.0
runs zero tests per mutant under vitest 5 and reports surviving mutants.
ESLint now ignores Stryker, coverage and report output.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

apps/web/vitest.config.ts

@@ -0,0 +1,27 @@
+import { fileURLToPath } from "node:url";
+import { defineConfig } from "vitest/config";
+
+const root = fileURLToPath(new URL(".", import.meta.url));
+
+export default defineConfig({
+  resolve: { alias: { "@": root } },
+  test: {
+    environment: "node",
+    include: ["src/**/*.spec.ts", "tests/unit/**/*.spec.ts"],
+    passWithNoTests: false,
+    coverage: {
+      provider: "v8",
+      // The functional core: pure logic lives in src/domain and must be fully covered.
+      include: ["src/domain/**/*.ts"],
+      exclude: ["src/domain/**/*.spec.ts"],
+      reporter: ["text", "json-summary", "html"],
+      reportsDirectory: "reports/coverage",
+      thresholds: {
+        lines: 100,
+        functions: 100,
+        branches: 100,
+        statements: 100,
+      },
+    },
+  },
+});