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.

vitest.snapshot.config.ts

@@ -36,8 +36,9 @@ if (process.env.DSH_SNAPSHOT === 'record') {
 
 export default defineConfig({
   // Same resolution note as vitest.config.ts: bare workspace names resolve
-  // through the root tsconfig paths map; the native option cannot do this.
-  plugins: [tsconfigPaths({ projects: ['./tsconfig.json'] })],
+  // through the tsconfig.base.json paths facade; the native option cannot do
+  // this (the root tsconfig is a solution file with no paths).
+  plugins: [tsconfigPaths({ projects: ['./tsconfig.base.json'] })],
   test: {
     setupFiles: ['./scripts/test-invariants.ts'],
     include: [