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.

app/androidApp/src/main/AndroidManifest.xml

@@ -12,12 +12,24 @@
         <activity
             android:name=".MainActivity"
             android:exported="true"
+            android:windowSoftInputMode="adjustResize"
             android:configChanges="orientation|screenSize|screenLayout|keyboardHidden">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />
             </intent-filter>
         </activity>
+
+        <!-- M4: serve cached media/documents to other apps (ACTION_VIEW). -->
+        <provider
+            android:name="androidx.core.content.FileProvider"
+            android:authorities="${applicationId}.fileprovider"
+            android:exported="false"
+            android:grantUriPermissions="true">
+            <meta-data
+                android:name="android.support.FILE_PROVIDER_PATHS"
+                android:resource="@xml/iris_file_paths" />
+        </provider>
     </application>
 
 </manifest>