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>

eslint.config.js

@@ -24,6 +24,9 @@ export default tseslint.config(
       "**/out/**",
       "**/cloned-websites/**",
       "**/public/showcase/**",
+      "**/.stryker-tmp/**",
+      "**/reports/**",
+      "**/coverage/**",
     ],
   },
   js.configs.recommended,