refactor(ext/vault): extract vault-form-wrapper.ts (Plan C Phase 4)

Moves renderFormWrapped (sticky save bar + header + dirty-state wiring), the
SAVE_HINT/isMac consts, and the __test__ export out of vault.ts into
vault-form-wrapper.ts, taking the VaultController ctx. Repoints the source-text
form-wrapper test to read the new module. No behavior change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

extension/src/vault/__tests__/form-wrapper.test.ts

@@ -4,7 +4,7 @@ import * as path from 'path';
 
 describe('fullscreen form dirty subtitle', () => {
   const vaultSrc = fs.readFileSync(
-    path.resolve(__dirname, '../vault.ts'),
+    path.resolve(__dirname, '../vault-form-wrapper.ts'),
     'utf-8',
   );