WIP: port deferred input/audio stubs to real SDL 3 APIs

Phase 7-8 of the migration. Replaces the per-subsystem stubs added in
the earlier "builds clean" commit with proper SDL 3 implementations.

- gfxengine: add SDL_Window* accessor window_handle() so callers that
  need to talk to SDL 3's window APIs (grab, text input) can reach it.
- states.cpp mouse grab: SDL_WM_GrabInput → SDL_SetWindowMouseGrab.
  Drops the local SDL_GRAB_* / SDL_WM_GrabInput stub macros.
- states.cpp text input: SDL_EnableUNICODE → SDL_StartTextInput /
  SDL_StopTextInput. kobo.cpp event loop now handles
  SDL_EVENT_TEXT_INPUT (in both quit_requested() and the main game
  loop) and forwards each byte of the UTF-8 string as a unicode value
  to gsm.press(-1, ch) so the high-score name-entry path keeps
  working without any state-side changes.
- kobo.cpp joystick: full re-port of init_js / close_js using
  SDL_GetJoysticks (returns SDL_JoystickID array), SDL_OpenJoystick,
  SDL_CloseJoystick. options.cpp joystick enumeration already used
  the new API from the previous commit.
- sound/audio.c: replace the silent stub with a real SDL 3 backend
  built on SDL_OpenAudioDeviceStream + an SDL_AudioStreamCallback.
  The engine's existing _audio_callback (Sint16 stereo fill-buffer)
  is kept; a small adapter (sdl3_stream_callback) runs it on a
  scratch buffer and pushes the result via SDL_PutAudioStreamData.
  audio_lock/unlock now call SDL_LockAudioStream / SDL_UnlockAudioStream.

After this commit the binary opens an audio device successfully and
the joystick path resolves real SDL 3 calls. Renderer + audio +
input are now all on SDL 3 with no compat shims.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

graphics/gfxengine.h

@@ -171,6 +171,7 @@ class gfxengine_t
 
 	/* Control */
 	cs_engine_t *cs()	{ return csengine; }
+	SDL_Window *window_handle()	{ return sdl_window; }
 	SDL_Surface *surface();
 	void flip();		// Flip pages
 	void update();		// Full update, making current draw page visible