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/protocol/frames.schema.json

@@ -68,7 +68,8 @@
       "error": { "payload": { "code": { "type": "string", "enum": ["auth", "not_found", "rate_limited", "media_too_large", "unsupported", "internal"] }, "message": { "type": "string" } } },
       "pong": { "payload": { "ts": { "type": "integer" } } },
       "sync.done": { "payload": { "cursor": { "type": "integer" } } },
-      "media.pull.end": { "payload": { "ok": { "type": "boolean" } } }
+      "media.pull.end": { "payload": { "ok": { "type": "boolean" } } },
+      "media.upload.ack": { "description": "Response to media.upload.end; ref is cached and usable in message.send media_refs.", "payload": { "ok": { "type": "boolean" }, "media_ref": { "type": "string" } } }
     },
     "app_to_server": {
       "hello": { "description": "First frame; auth + caps.", "payload": { "token": { "type": "string" }, "device_id": { "type": "string" }, "device_name": { "type": "string" }, "caps": { "type": "object", "properties": { "min_protocol": {"type":"integer"}, "media": {"type":"boolean"}, "push": {"type":"string"} } }, "fcm_token": { "type": "string" }, "ntfy_topic": { "type": "string" } } },