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.

.gitignore

@@ -20,3 +20,9 @@
 /packaging/pkg/
 /packaging/src/
 /packaging/*.pkg.tar.zst
+
+# iOS generated state: XcodeGen owns the project and the Info.plist, and
+# scripts/build-ios.sh writes the bindings and DerivedData under ios/build.
+/ios/build/
+/ios/*.xcodeproj/
+/ios/ProtonStream/Info.plist