M4: media upload/download/playback (both directions)

Gateway plugin:
- media.upload (chunked binary) -> size/sha256 verify + MIME re-sniff ->
  cache_*_from_bytes -> media.upload.ack
- media.offer / media.pull (chunked) for agent-sent media, delivery-path
  security re-checked at pull time
- send_* overrides mint media_id and emit media.offer
- message.send media_refs resolve to cached inbound media
- per-send + per-chunk timeouts so a stalled peer can't starve the rest

App (Kotlin CMP):
- Protocol: media frame types/payloads/builders
- GatewayClient: binary session, uploadMedia (chunked + streaming sha256),
  pullMedia serialized via Mutex so concurrent offers don't interleave
- ChatStore/IrisController: MediaItem, attachments, auto-pull on offer
- Platform media: SAF picker, ExoPlayer (audio mini-player + video), image
  loader, FileProvider document open (Android); AWT-free desktop actuals
- ChatScreen: attach button + chips, media rendering, keyboard dismiss on send

UI polish:
- preserve image aspect ratio (no stretching), cap dominant dimension
- adjustResize so only chat content squeezes for the keyboard
- clear focus (hide keyboard) on send

Docs: media.upload.ack in 04-wire-protocol.md + frames.schema.json +
07-media.md; M4 marked complete in 14-milestones.md.

Tests: 17-test tests/gateway/test_android.py suite passes.

docs/04-wire-protocol.md

@@ -227,6 +227,13 @@ See `07-media.md`.
 {"type":"media.upload.end","id":11,"payload":{"media_ref":"mu_1","sha256":"…"}}
 ```
 
+### `media.upload.ack`
+Server → App response to `media.upload.end`: the ref is cached and may now be
+referenced in a `message.send` `media_refs`. Failures use `error` frames instead.
+```json
+{"type":"media.upload.ack","id":11,"payload":{"ok":true,"media_ref":"mu_1"}}
+```
+
 ### `media.pull`
 Request agent-sent media bytes.
 ```json