feat(ios): audio overhaul, Join/Leave Voice, channel-id sync fix, stereo mic capture

Three iOS client problems fixed plus a new core stereo-mic capture ABI:

1. Channel-id sync bug (mic button permanently dimmed): SessionState never
   synced currentChannelId from the self user's channelId on connect, so the
   mic button (gated on currentChannelId == 0) stayed dimmed. Added
   syncSelfChannel() (mirrors macOS MainWindowController.swift:461,491,522);
   called from init/.channelList/.userJoined/.userLeft/.userUpdated/.joinResult.
   Added applyServerMuteState() + serverMuted/serverDeafened to VoiceState.

2. Join/Leave Voice button: replaced icon-only mic toggle with explicit
   text button (parity with macOS). Mute/deafen disable when not in voice.

3. IOSAudioRouter.swift (new): full AVAudioSession routing layer — input
   port selection, built-in mic orientation/polar patterns, Bluetooth
   HFP/A2DP/Off modes, Standard/Raw mic processing, stereo capture, AirPlay,
   UserDefaults persistence. AudioSessionManager delegates to it.

4. Core stereo-mic capture (append-only ABI): vc_set_capture_channels()
   lets the core open the mic device in stereo (2-ch interleaved). LocalStream
   gains capture_channels; ensure_audio_running reads it; audio_engine.cpp
   capture_accum_ + on_capture updated to channel-aware accumulation. Test
   test_stereo_mic_capture (headless, L!=R stereo round-trip). Swift wrapper
   VoiceCatClient.setCaptureChannels.

5. Settings UI rework: AVAudioSession-derived input/output tree replaces
   miniaudio device picker.

6. iOS deployment target raised to 18.0 (Package.swift + project.pbxproj).
   swift-tools-version 6.0 with swiftLanguageModes .v5.

Docs: tech-stack.md, architecture.md, voice.md, roadmap.md, building.md
updated; stale 'vc_audio_suspend/resume deferred' claims corrected.

Verified: ctest --preset dev 21/21 green; swift test 6/6 green;
xcodebuild -target VoiceCatiOS -sdk iphonesimulator BUILD SUCCEEDED.

docs/architecture.md

@@ -136,7 +136,7 @@ Design notes:
 
 ### Per-platform binding notes
 
-- **Swift / Apple.** Import the C ABI via a **module map** (`module VoiceCatC { header "voicecat.h" }`) staged into the XCFramework headers by `clients/apple/scripts/build-xcframework.sh` — Swift gets a clean `import VoiceCatC` with all C enums/structs/functions available directly (no manual redeclaration, unlike the C# P/Invoke layer). A **Swift wrapper** (`VoiceCatCore` package at `clients/apple/`) provides Swift-idiomatic types (`VoiceCatResult`, `VoiceCatEvent`, `Channel`, `User`, etc.) on top, mirroring the C# `VoiceCat.Interop` layer. Callbacks use `@convention(c)` closures (plain C function pointers, not ARC-managed closures) + `Unmanaged.passUnretained(self)` as the `user` context (the Swift analog of C#'s `[UnmanagedCallersOnly]` + `GCHandle`). Events are delivered on `@MainActor` via a coalesced `DispatchQueue.main` drain (one async block scheduled at a time) — the Swift analog of C#'s `Channel<VoiceCatEvent>` + 30ms WinForms Timer pump. `deinit` calls `vc_client_destroy` (joins all threads) then frees native CString config storage (the core stores raw pointers, doesn't copy). **macOS UI: AppKit** (chosen over SwiftUI for the most mature VoiceOver accessibility story — same rationale as the Windows client's WinForms choice); **iOS UI: SwiftUI** (narrower control surface, sufficient VoiceOver support). On **iOS** the app must still own `AVAudioSession` (category `.playAndRecord`, `.voiceChat` mode), request mic permission, and handle interruptions/route changes — the core exposes hooks (`vc_audio_suspend`/`vc_audio_resume`) the Swift layer calls from `AVAudioSession` notifications (these hooks are deferred until the iOS client milestone to keep the ABI stable). Background voice and VoIP push (CallKit/PushKit) are a later milestone. The XCFramework carries a **fat static library** (`libvoicecat-fat.a`) bundling `libvoicecat.a` + all vcpkg static deps so the Swift Package links a single self-contained `.a` per slice.
+- **Swift / Apple.** Import the C ABI via a **module map** (`module VoiceCatC { header "voicecat.h" }`) staged into the XCFramework headers by `clients/apple/scripts/build-xcframework.sh` — Swift gets a clean `import VoiceCatC` with all C enums/structs/functions available directly (no manual redeclaration, unlike the C# P/Invoke layer). A **Swift wrapper** (`VoiceCatCore` package at `clients/apple/`) provides Swift-idiomatic types (`VoiceCatResult`, `VoiceCatEvent`, `Channel`, `User`, etc.) on top, mirroring the C# `VoiceCat.Interop` layer. Callbacks use `@convention(c)` closures (plain C function pointers, not ARC-managed closures) + `Unmanaged.passUnretained(self)` as the `user` context (the Swift analog of C#'s `[UnmanagedCallersOnly]` + `GCHandle`). Events are delivered on `@MainActor` via a coalesced `DispatchQueue.main` drain (one async block scheduled at a time) — the Swift analog of C#'s `Channel<VoiceCatEvent>` + 30ms WinForms Timer pump. `deinit` calls `vc_client_destroy` (joins all threads) then frees native CString config storage (the core stores raw pointers, doesn't copy). **macOS UI: AppKit** (chosen over SwiftUI for the most mature VoiceOver accessibility story — same rationale as the Windows client's WinForms choice); **iOS UI: SwiftUI** (narrower control surface, sufficient VoiceOver support). On **iOS** the app owns `AVAudioSession` (category `.playAndRecord`), requests mic permission, and handles interruptions/route changes — the core exposes hooks (`vc_audio_suspend`/`vc_audio_resume`, implemented) the Swift layer calls from `AVAudioSession` notifications. All iOS audio routing (input port selection, mic orientation/polar patterns, HFP vs A2DP, measurement/raw mode, stereo capture) is driven from the Swift `IOSAudioRouter` singleton via `AVAudioSession` *before* the core (miniaudio) opens its device — miniaudio does NOT touch `AVAudioSession` on iOS. The core is told the capture channel count via `vc_set_capture_channels` (append-only ABI). iOS 18.0 deployment target. Background voice and VoIP push (CallKit/PushKit) are a later milestone. The XCFramework carries a **fat static library** (`libvoicecat-fat.a`) bundling `libvoicecat.a` + all vcpkg static deps so the Swift Package links a single self-contained `.a` per slice.
 - **iOS screen / system-audio sharing** is supported via a **ReplayKit Broadcast Upload
   Extension** (the same mechanism Discord uses; triggered from Control Center's screen-record
   button via `RPSystemBroadcastPickerView`). The extension receives