feat: LAN Wi-Fi casting to Chromecast via local HTTP relay

Re-serve decrypted fMP4 video over HTTP on the LAN so a Chromecast can
play the stream. The relay runs in the Electron main process — same trust
boundary as downloads and MSE playback.

- cast-relay.js: HTTP server with BoxAccumulator (reassembles WebRTC
  chunks into moof+mdat pairs), ring buffer, backpressure, finish() for
  clean end-of-stream, fixed port range 19550-19553
- cast-chromecast.js: mDNS discovery (bonjour-service) + CASTV2 protocol
  (castv2-client), connect/reload/disconnect lifecycle
- Seek-aware: relay restarts on every seek, Chromecast reloads new URL;
  generation counter prevents stale async errors from killing active
  restarts; landingPlayheadRef suppresses programmatic seeking events
- Device picker in video top bar with scan, device selection, copy-URL
  fallback, and cast status indicator
- IPC bridge (main/preload/platform) for start/push/stop/finish/status/
  discover/chromecastConnect/chromecastReload/chromecastDisconnect
- Phase 3 design doc for DLNA/Smart TV in docs/cast-smart-tv.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

1 files changed, 53 insertions, 2 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/platform.js b/packages/meshbay-hub/src/meshbay_hub/static/platform.js
index 02107f5..d400b4e 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/platform.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/platform.js
@@ -46,6 +46,8 @@ export const capabilities = {
   localFolders: Boolean(bridge && bridge.capabilities && bridge.capabilities.localFolders),
   // A real save dialog and a write that does not pass through the page.
   nativeSave: Boolean(bridge && bridge.capabilities && bridge.capabilities.nativeSave),
+  // Cast decrypted video to a device on the same LAN via a local HTTP relay.
+  lanCast: Boolean(bridge && bridge.capabilities && bridge.capabilities.lanCast),
 };
 
 /**
@@ -234,8 +236,56 @@ export const node = {
   },
 };
 
+/**
+ * LAN cast relay — re-serve decrypted video segments over HTTP so a
+ * Chromecast or Smart TV on the same Wi-Fi can play the stream.
+ *
+ * Absent in a browser, where the relay cannot run: there is no main process
+ * to bind a server socket in, and a page cannot open one.
+ */
+export const cast = {
+  available: Boolean(bridge && bridge.cast),
+  async start(opts) {
+    if (!bridge || !bridge.cast) return null;
+    return bridge.cast.start(opts);
+  },
+  async push(data) {
+    if (!bridge || !bridge.cast) return false;
+    return bridge.cast.push(data);
+  },
+  async stop() {
+    if (!bridge || !bridge.cast) return false;
+    return bridge.cast.stop();
+  },
+  async finish() {
+    if (!bridge || !bridge.cast) return false;
+    return bridge.cast.finish();
+  },
+  async status() {
+    if (!bridge || !bridge.cast) return null;
+    return bridge.cast.status();
+  },
+  async discover() {
+    if (!bridge || !bridge.cast) return [];
+    return bridge.cast.discover();
+  },
+  async chromecastConnect(opts) {
+    if (!bridge || !bridge.cast) return null;
+    return bridge.cast.chromecastConnect(opts);
+  },
+  async chromecastReload(opts) {
+    if (!bridge || !bridge.cast) return null;
+    return bridge.cast.chromecastReload(opts);
+  },
+  async chromecastDisconnect() {
+    if (!bridge || !bridge.cast) return false;
+    return bridge.cast.chromecastDisconnect();
+  },
+};
+
 export default { isNative, hubBase, capabilities, secrets, nativeSave,
-                 apiFetch, device, bridgeMessage, folder, rootPicker, node };
+                 apiFetch, device, bridgeMessage, folder, rootPicker, node,
+                 cast };
 
 // Also a global, because `transport.js` is loaded as a classic script — it
 // predates the module graph and exposes `MeshBayTransport` the same way. The
@@ -244,5 +294,6 @@ export default { isNative, hubBase, capabilities, secrets, nativeSave,
 if (typeof window !== 'undefined') {
   window.MeshBayPlatform = { isNative, hubBase, capabilities, secrets,
                              nativeSave, apiFetch, device,
-                             bridgeMessage, folder, rootPicker, node };
+                             bridgeMessage, folder, rootPicker, node,
+                             cast };
 }