feat(ios): a sideloadable iOS client over the shared Rust engine

A SwiftUI app that drives the same UniFFI engine as Android: Rust owns the
Proton sessions, SQLite and every decrypted byte, and Swift asks it questions
off the main thread. Playback goes through MPVKit's libmpv, with the pstr://
stream protocol ported from the Android JNI half, and secrets live in the
Keychain.

scripts/build-ios.sh builds the static library for aarch64-apple-ios, writes
the Swift bindings (module PstrBridge), generates the Xcode project with
XcodeGen and packages an unsigned .ipa for SideStore or AltStore, which
re-sign it with the viewer's own Apple ID. The generated project, Info.plist
and build output are ignored.

ios/ProtonStream/Bridge/ProtonStream-Bridging-Header.h

@@ -0,0 +1,4 @@
+// C declarations Swift sees without a module import: the UniFFI scaffolding
+// generated beside PstrBridge.swift, and the stream ABI UniFFI does not cover.
+#include "PstrBridgeFFI.h"
+#include "pstr_stream.h"