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/14-milestones.md

@@ -90,17 +90,31 @@ has explicit **acceptance criteria**. Work top-to-bottom; don't skip M0/M1.
 
 ## M4 — Media
 **Goal:** attach + receive + play media.
-- [ ] Inbound: `media.upload` chunked → `cache_*_from_bytes` → `media_urls`;
+- [X] Inbound: `media.upload` chunked → `cache_*_from_bytes` → `media_urls`;
       size limit + sha256 + MIME re-sniff.
-- [ ] Outbound: `send_*` → `media.offer`; `media.pull` chunked; delivery-path
+- [X] Outbound: `send_*` → `media.offer`; `media.pull` chunked; delivery-path
       security.
-- [ ] App: SAF pickers + preview chips + upload; `media.pull` → cache;
+- [X] App: SAF pickers + preview chips + upload; `media.pull` → cache;
       **ExoPlayer** inline (audio mini-player, video fullscreen/PiP); image/doc
       viewers.
 - **Demo (on-device):** attach a photo + video (agent sees them); ask agent to
   send an image/video → plays live in-app.
 - **Accept:** both directions work; over-limit rejected; playback is live;
   only allowed files are servable.
+- **Status (complete):** Both directions verified end-to-end on the MIX 2S.
+  Inbound: SAF-picked photo → chunked binary `media.upload` (256 KiB) →
+  gateway size+sha256 verify + MIME re-sniff → `cache_image_from_bytes` →
+  `media.upload.ack` → `message.send` with `media_refs` → agent vision
+  described the image accurately. Outbound: agent `send_image` → `media.offer`
+  → app auto-`media.pull` (chunked) → cache → image rendered inline. Live
+  playback verified on-device: agent offered a 2s MP4 + 2s MP3 → app pulled
+  both → ExoPlayer video player (blue frame, 00:02/00:02, controls) + audio
+  mini-player rendered and playable. Over-limit rejection, sha256 mismatch,
+  and delivery-path security are covered by the 17-test
+  `tests/gateway/test_android.py` suite (all pass). Note: the app serializes
+  pulls (single `binarySession` slot) via a `Mutex` so concurrent offers don't
+  interleave their byte streams. `media.upload.ack` documented in
+  `04-wire-protocol.md` + `frames.schema.json`.
 
 ## M5 — Push + offline (FCM + ntfy)
 **Goal:** reach the phone when backgrounded; catch up on reconnect.