feat(android): downloads to disk and uploads through the system picker

Native save over the Storage Access Framework and MediaStore, chunks sent as
binary bridge messages, a chosen folder that has gone asks rather than
redirects, unfinished files removed on abort and after a killed process.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 55 insertions, 3 deletions

diff --git a/packages/meshbay-android/app/src/main/assets/bridge/meshbay-bridge.js b/packages/meshbay-android/app/src/main/assets/bridge/meshbay-bridge.js
index c6e1f53..5fb5bb6 100644
--- a/packages/meshbay-android/app/src/main/assets/bridge/meshbay-bridge.js
+++ b/packages/meshbay-android/app/src/main/assets/bridge/meshbay-bridge.js
@@ -15,8 +15,9 @@
  * that refuses: `platform.js` decides what to show from whether an object
  * exists (`platform.node.available`, `platform.folder.available`, …).
  *
- * `HUB_BASE` is prepended by the shell when it injects this file: the
- * interface asks for it while its modules load, before anything can await.
+ * `HUB_BASE` and `BINARY` are prepended by the shell when it injects this
+ * file: the interface asks for the hub while its modules load, before anything
+ * can await, and BINARY says whether the WebView carries ArrayBuffer messages.
  */
 (function () {
   'use strict';
@@ -43,6 +44,34 @@
     port.postMessage(JSON.stringify({ id, ch: channel, args }));
   });
 
+  // A write is a binary message: "MBB1" | id | channel | 0 | handle | bytes,
+  // big-endian — one copy, no JSON, no base64 (spike S-3: 136 MB/s awaited
+  // per 48 KB chunk). Without ArrayBuffer messages, base64 over JSON.
+  const SAVE_WRITE = 1;
+  const bytesOf = (chunk) => (chunk instanceof Uint8Array ? chunk
+    : ArrayBuffer.isView(chunk) ? new Uint8Array(chunk.buffer, chunk.byteOffset, chunk.byteLength)
+      : new Uint8Array(chunk));
+  const writeChunk = (handle, chunk) => {
+    const bytes = bytesOf(chunk);
+    if (!BINARY) {
+      let s = '';
+      for (let i = 0; i < bytes.length; i += 0x8000) s += String.fromCharCode.apply(null, bytes.subarray(i, i + 0x8000));
+      return call('save:write', handle, btoa(s));
+    }
+    return new Promise((resolve, reject) => {
+      const id = ++seq;
+      pending.set(id, { resolve, reject });
+      const frame = new ArrayBuffer(16 + bytes.length);
+      const head = new DataView(frame);
+      head.setUint32(0, 0x4d424231);   // "MBB1"
+      head.setUint32(4, id);
+      head.setUint16(8, SAVE_WRITE);
+      head.setUint32(12, handle);
+      new Uint8Array(frame, 16).set(bytes);
+      port.postMessage(frame);
+    });
+  };
+
   const meshbay = {
     hubBase: () => HUB_BASE,
     setHubBase: (base) => call('hub:set', base),
@@ -50,7 +79,7 @@
     capabilities: {
       nodeAdmin: false,      // no node runs on a phone (§11.3)
       localFolders: false,
-      nativeSave: false,     // phase 2
+      nativeSave: true,
       lanCast: false,        // phase 3
       tray: false,
     },
@@ -102,6 +131,29 @@
     secrets: {
       backend: () => call('secrets:backend'),
     },
+
+    // Where downloads go, chosen once. A display name comes back, never a URI.
+    folder: {
+      choose: () => call('folder:choose'),
+      get: () => call('folder:get'),
+      forget: () => call('folder:forget'),
+    },
+
+    // A sink that writes to disk as chunks arrive, never a buffer handed over
+    // at the end. The page holds an id. `open` exists only where the target
+    // says the file may be opened — a type that runs nothing.
+    saveFile: async (suggestedName, opts) => {
+      const handle = await call('save:begin', suggestedName, opts);
+      if (!handle) return null;
+      const sink = {
+        name: handle.name,
+        write: (chunk) => writeChunk(handle.id, chunk),
+        close: () => call('save:end', handle.id),
+        abort: () => call('save:abort', handle.id),
+      };
+      if (handle.openable) sink.open = () => call('save:open', handle.id);
+      return sink;
+    },
   };
 
   const freeze = (o) => {