feat(sfu): ship the prebuilt sidecar binary in the package (true plug-and-play)

Backend-hosted rooms now work on a plain `composer require`/`update` with no
download step: rust/dist/blax-webrtc-sfu-linux-x86_64 (+ .sha256) is committed
and BinaryManager::find() resolves rust/dist/blax-webrtc-sfu-<platform> before
the download fallback. A consuming app just sets WEBRTC_MEDIA_ENGINE=
RustMediaEngine (+ pin/publish WEBRTC_SFU_UDP_PORT and WEBRTC_SFU_PUBLIC_IP for
prod) and the sidecar auto-spawns.

- .gitignore: keep rust/target/ ignored, COMMIT rust/dist/ (shipped binaries)
- BinaryManager: add shipped-binary resolution + doc; README/doc updated

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -7,6 +7,8 @@ composer.lock
 .idea/
 .vscode/
 
-# Rust build artifacts (Cargo.lock IS committed — bin crate, reproducible releases)
+# Rust build artifacts (Cargo.lock IS committed — bin crate, reproducible releases).
+# rust/target/ is local build output. rust/dist/ IS committed: it holds the
+# prebuilt release binaries (+ .sha256) SHIPPED with the package for plug-and-play
+# (BinaryManager::find resolves rust/dist/blax-webrtc-sfu-<platform> — no download).
 /rust/target/
-/rust/dist/

rust/dist/blax-webrtc-sfu-linux-x86_64.sha256

@@ -0,0 +1 @@
+79840ef020c334d9bdab5fcf5f72be918d444a6bdc6e9236574bd019357672b5  blax-webrtc-sfu-linux-x86_64

src/Media/Rust/BinaryManager.php

@@ -45,7 +45,7 @@ class BinaryManager
         return (string) ($this->config['version'] ?? self::VERSION);
     }
 
-    /** First usable binary (override → dev build → installed), or null. */
+    /** First usable binary (override → dev build → shipped → installed), or null. */
     public function find(): ?string
     {
         $packageRoot = dirname(__DIR__, 3);
@@ -54,6 +54,11 @@ class BinaryManager
             $this->config['binary'] ?? null,
             $devBuilds ? $packageRoot.'/rust/target/release/blax-webrtc-sfu' : null,
             $devBuilds ? $packageRoot.'/rust/target/debug/blax-webrtc-sfu' : null,
+            // Plug-and-play: the prebuilt binary SHIPPED with the package (composer
+            // installs it alongside the PHP), resolved per platform. This is what
+            // makes backend-hosted rooms work on `composer require`/`update` with
+            // no download step — see rust/dist/ (committed).
+            $packageRoot.'/rust/dist/blax-webrtc-sfu-'.$this->platform(),
             $this->installPath(),
         ]);