Replace C Tor with Rust Arti for Tor integration

- Replace C Tor (0.4.8.21) with Rust Arti (1.9.0/arti-client 0.38)
- 70% smaller binary: 21MB xcframework vs 67MB (6.9MB vs 14MB per slice)
- Memory-safe Rust implementation with modern async (tokio)
- Same SOCKS5 proxy interface at 127.0.0.1:39050 for drop-in compatibility
- FFI wrapper (arti-bitchat crate) with C-compatible exports
- Swift TorManager maintains identical public API
- Aggressive size optimization: opt-level=z, lto=fat, panic=abort, strip
- Supports iOS device, iOS simulator (Apple Silicon), and macOS

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

localPackages/Arti/Sources/TorURLSession.swift

@@ -0,0 +1,63 @@
+import Foundation
+#if os(macOS)
+import CFNetwork
+#endif
+
+/// Provides a shared URLSession that routes traffic via Tor's SOCKS5 proxy
+/// when Tor is enforced/ready. Allows swapping between proxied and direct
+/// sessions so UI can toggle Tor usage at runtime.
+public final class TorURLSession {
+    public static let shared = TorURLSession()
+
+    // Default (no proxy) session for direct Nostr access when Tor is disabled.
+    private var defaultSession: URLSession = TorURLSession.makeDefaultSession()
+
+    // Proxied (SOCKS5) session that routes through Tor.
+    private var torSession: URLSession = TorURLSession.makeTorSession()
+    private var useTorProxy: Bool = true
+
+    public var session: URLSession {
+        useTorProxy ? torSession : defaultSession
+    }
+
+    // Recreate sessions so new clients bind to the fresh SOCKS/control ports after a Tor restart.
+    public func rebuild() {
+        defaultSession = TorURLSession.makeDefaultSession()
+        torSession = TorURLSession.makeTorSession()
+    }
+
+    public func setProxyMode(useTor: Bool) {
+        guard useTorProxy != useTor else { return }
+        useTorProxy = useTor
+        rebuild()
+    }
+
+    private static func makeTorSession() -> URLSession {
+        let cfg = URLSessionConfiguration.ephemeral
+        cfg.waitsForConnectivity = true
+        // Keep in sync with TorManager defaults
+        let host = "127.0.0.1"
+        let port = 39050
+        #if os(macOS)
+        cfg.connectionProxyDictionary = [
+            kCFNetworkProxiesSOCKSEnable as String: 1,
+            kCFNetworkProxiesSOCKSProxy as String: host,
+            kCFNetworkProxiesSOCKSPort as String: port
+        ]
+        #else
+        // iOS: CFNetwork SOCKS proxy keys are unavailable at compile time.
+        cfg.connectionProxyDictionary = [
+            "SOCKSEnable": 1,
+            "SOCKSProxy": host,
+            "SOCKSPort": port
+        ]
+        #endif
+        return URLSession(configuration: cfg)
+    }
+
+    private static func makeDefaultSession() -> URLSession {
+        let cfg = URLSessionConfiguration.default
+        cfg.waitsForConnectivity = true
+        return URLSession(configuration: cfg)
+    }
+}