Fix Settings plugin installs registering unloadable directory paths

Plugins installed from Settings → Built-in Plugins registered the
manifest directory as the plugin path, but since FN-4128 the loader
requires a loadable entry FILE (Node ESM cannot import directories),
so enabling failed with "Plugin entry must be a file, got directory".
Only the CLI startup path had been migrated to entry-file resolution,
which is why CLI-auto-installed plugins worked and Settings installs
never did.

- Add resolvePluginEntryPath (bundled.js → dist/index.js →
  src/index.ts) to @fusion/core; the CLI keeps its local copy (its
  test fs mocks don't reach externalized core) with sync comments
  both ways.
- Register the resolved entry file in both dashboard install routes;
  400 with a clear message when a package has no loadable entry.
- Heal legacy directory-path registrations on enable, mirroring the
  CLI's startup heal, so existing broken rows recover from the UI
  without a restart.
- Route tests: assert installs register entry files, cover the
  enable-route heal, and update existing install tests to the
  entry-file contract.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/cli/src/plugins/bundled-plugin-install.ts

@@ -78,6 +78,9 @@ function resolveBundledPluginDir(pluginId: string): string | null {
  * Returns null when the directory exists but none of the loadable entry files
  * are present. Callers must treat that as a missing bundle rather than
  * persisting a directory path that Node cannot import.
+ *
+ * Keep in sync with resolvePluginEntryPath in @fusion/core (plugin-loader.ts),
+ * which the dashboard install/enable routes use for the same contract.
  */
 export function resolvePluginEntryPath(pluginDir: string): string | null {
   const candidates = [