sitesmith: strip diagnostic shim + state-dump now that the fix is verified

Apply path is stable end-to-end with the linkedNode pre-creation patch;
diagnostic shim + window.__sitesmithLastState dump are no longer earning
their footprint. Reverts:
  - vite.config.ts: drops the tiny-invariant alias
  - src/utils/tiny-invariant-shim.ts: deleted
  - PageContext.tsx: removes the post-walk dump/scan block

craft/vite.config.ts

@@ -8,9 +8,6 @@ export default defineConfig({
   resolve: {
     alias: {
       '@': path.resolve(__dirname, './src'),
-      // Diagnostic: replace tiny-invariant with a shim that always logs the
-      // failure message (the upstream prod build strips it).
-      'tiny-invariant': path.resolve(__dirname, './src/utils/tiny-invariant-shim.ts'),
     },
   },
   build: {