refactor(vitest): resolve every lane through the tsconfig.base.json facade

tsconfig.vitest.json is deleted; its job (a paths map applying to all
test files) is inherent in tsconfig.base.json having no include —
vite-tsconfig-paths treats that as match-all. All four vitest configs
now pin the same facade: the unit config gains a shared pathsPlugin()
helper, the web lane drops its handwritten webserver alias (apps/web
tests are covered by match-all), and the snapshot lane leaves the root
solution (which no longer carries paths) and stops resolving client
imports through package exports. tsconfig.base.json documents the
facade role and bans include/files; the CI eslint cache key hashes the
four graph tsconfigs; the eslint tests-block comment states resolution
via the solution to tsconfig.host.json.

Per missions/tsconfig-single-graph-migration.md §3.

eslint.config.mjs

@@ -86,7 +86,8 @@ export default tseslint.config(
     languageOptions: {
       parserOptions: {
         // Same shared project service as the src block: test files resolve
-        // through the root tsconfig (its include covers every tests/ tree).
+        // through the root solution to tsconfig.host.json (its include covers
+        // every host tests/ tree).
         projectService: true,
         tsconfigRootDir: import.meta.dirname,
       },