fix: comprehensive bug fixes across codebase (~100 bugs)

Critical fixes:
- Fix CallObjectMethod/CallStaticObjectMethod/etc returning non-nil
  *Object for Java null -- template-level fix, callers can now use
  nil checks correctly
- Fix NewManager double-GlobalRef leak: GetSystemService already
  returns GlobalRef, don't wrap again (template fix, 34 managers)
- Fix ContextFromObject Close() double-free: now creates own GlobalRef
- Fix ExtractBundle returning unusable Bundle (nil VM field)
- Fix proxy doProxyInit leaking GlobalRefs on partial failure
- Fix SetProxyClassLoader race condition (now mutex-protected)

env.go nil-return fixes (template-level):
- All Object-returning JNI functions now return nil for Java null
  instead of non-nil *Object with zero ref

Local ref leak fixes:
- Intent PutStringExtra/PutIntExtra/PutBoolExtra: free jKey/jVal/cls
- Intent NewIntent: free cls and original local ref after NewGlobalRef
- Notification NewChannel/NewBuilder: free string local refs
- PDF NewCanvas/NewPaint: free original local ref after NewGlobalRef
- Bluetooth NewAdapter: free BluetoothManager GlobalRef after use
- ARGB8888/MonospaceTypeface: cache with sync.Once to avoid leak

Error handling fixes:
- Intent GetBoolExtra now returns error instead of swallowing it
- proxy_invoke: add ExceptionCheck after getName() call
- proxy_invoke: delete 'other' local ref in equals branch

Code quality fixes:
- Remove initSkipped dead code from all 59 init.go files (template)
- Remove ReturnConversionCode dead function
- Merge: don't mutate input spec's IntentExtras slice
- specgen: escape string constant values with strconv.Quote

Example fixes:
- Add ObjectFromPtr to avoid capi imports in all 53 examples
- Add sync.Mutex for outputBuf race condition in ui.go
- Add SCHEDULE_EXACT_ALARM permission for alarm example
- Fix app_framework ignoring SetAction error
- Add defer mgr.Close() for usage example

Author: xaionaro@dx.center