Implement Cast Streaming mirroring, DLNA casting, daemon+GUI, and breadd integration

Builds out the full v1 scope: a vendored+patched openscreen subset for
low-latency Cast Streaming (Mirroring receiver 0F5096E8) alongside the
existing Cast V2/HLS and new DLNA/AVTransport casting paths, breadcastd's
Idle/Casting state machine with a private IPC socket, the breadcast GTK4
popup as a thin IPC client, and bread.cast.*/bread.command.cast.* breadd
integration (device discovery, start/stop, mirroring lifecycle events).
Also adds bakery/systemd/Forgejo CI packaging.

Validated end-to-end against a real Chromecast/Google TV: negotiated
Cast Streaming session, live pipeline playback, and daemon+GUI click-to-cast/
stop through the actual popup.

EVENTS.md

@@ -0,0 +1,44 @@
+# breadcast — bread event integration
+
+breadcast is a standalone screen-mirroring app: it works exactly the same
+with or without `breadd` running. When breadd *is* present, `breadcastd`
+publishes events into the shared bread automation fabric and listens for a
+small set of commands. See the parent `bread` repo's `Documentation.md` —
+specifically its "Namespaces" and "Integrating a bread\* app" sections — for
+the general convention this follows.
+
+App id: **`cast`**. Transport: `bread-utils`'s `bread_client` module
+(feature `bread-client`) — `breadcastd` links it directly, since it's a
+long-running process for both the command-subscription half and the
+discovery-driven emit half.
+
+## Events published (`bread.cast.*`)
+
+| Event | Data | When |
+|-------|------|------|
+| `bread.cast.device_found` | `{ "id": "<mdns id or DLNA description URL>", "name": "<friendly name>", "model": "<model>", "protocol": "cast" \| "dlna" }` | A Chromecast/Google TV (mDNS) or DLNA/UPnP media renderer (SSDP) is discovered (or re-resolved) on the LAN. Fires on every re-resolution, not just the first sighting — treat it as an upsert keyed by `id`, not an append-only log. `id` is protocol-specific and only unique *within* a protocol — a Cast device's mDNS id and a DLNA device's description URL share no namespace. |
+| `bread.cast.mirroring_started` | `{ "device_id": "<id>", "device_name": "<name>", "protocol": "cast" \| "dlna" }` | A mirroring session successfully started, whether triggered by `bread.command.cast.start`, the `breadcast` GTK popup, or (once wired) any other IPC client. |
+| `bread.cast.mirroring_stopped` | `{}` | A mirroring session ended, whether via an explicit stop (command, IPC, or GTK popup) or unprompted (the portal picker's "stop sharing", the receiver dropping the connection, a DLNA renderer stopping playback from its own remote). There is no separate "stopped by whom" distinction in this event — `breadcastd`'s own logs have that detail if needed. |
+| `bread.cast.mirroring_failed` | `{ "device_id": "<id>", "error": "<message>" }` | A `start_cast`/`bread.command.cast.start` attempt failed before a session was established (device unreachable, portal capture denied, negotiation timeout, renderer rejected the stream, etc). |
+
+## Commands honored (`bread.command.cast.*`)
+
+| Command | Data | Effect |
+|---------|------|--------|
+| `bread.command.cast.start` | `{ "device_id": "<id>" }` | Starts mirroring to the given device (looked up across both the Cast and DLNA device lists — same `device_id` a `list_devices`/`device_list_changed` payload reports). Fire-and-forget: the outcome shows up as `bread.cast.mirroring_started`/`.failed`, not a reply to this command. No-ops (logged) if already casting. |
+| `bread.command.cast.stop` | none | Stops the active mirroring session, if any. No-ops if already idle. |
+
+Both commands are handled identically regardless of whether the session was
+started from here, the `breadcast` GTK popup, or the IPC socket directly —
+`breadcastd`'s daemon actor has exactly one notion of "the active session"
+(see `breadcastd/src/daemon.rs`), not one per control surface.
+
+## Fail-safe behavior
+
+- If breadd isn't installed or isn't running, `emit` is a silent no-op
+  (`BreadClient::emit` never blocks or errors the caller) and the
+  command subscription simply never receives anything — breadcastd's actual
+  discovery functionality is entirely unaffected either way.
+- If breadd restarts, the command subscription reconnects automatically
+  (`BreadClient::subscribe`'s background thread has its own backoff loop);
+  no restart of breadcastd is needed.