SPM: wire local plugins correctly — add Package.swift to each plugin's files array

Root cause of the SPM runtime regressions (call/chat push when closed, photo save):
our local file: plugins didn't function because their package.json `files` array
omitted "Package.swift", so npm can drop it from node_modules on install → cap sync
can't wire the plugin into the CapApp-SPM package → the plugin isn't loaded at
runtime. Official plugins all list Package.swift in files; ours now do too.

Also add build-log diagnostics (node_modules symlink/copy + Package.swift presence,
and the generated CapApp-SPM/Package.swift) so the wiring is provable, not guessed.

Reapplies the SPM migration (Step B) that was reverted.

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

mobile/plugins/media-library/package.json

@@ -10,7 +10,8 @@
   "files": [
     "dist/",
     "ios/",
-    "MediaLibrary.podspec"
+    "MediaLibrary.podspec",
+    "Package.swift"
   ],
   "capacitor": {
     "ios": {