windows(desktop): gate coreaudio / VoiceProcessingIO to macOS-only targets

First step of the Windows x86_64 desktop build: stop pulling
coreaudio-rs into the Windows dependency graph so the project can at
least run `cargo check --target x86_64-pc-windows-msvc`. Software AEC
is already disabled in engine.rs so there's nothing else to stub — the
macOS-specific VPIO path is skipped via #[cfg(target_os = "macos")] on
both sides and Windows falls through to the plain CPAL
AudioCapture/AudioPlayback branch that already existed.

crates/wzp-client/Cargo.toml
  - coreaudio-rs optional dep moved under [target.'cfg(target_os = "macos")']
  - `vpio` feature now uses `dep:coreaudio-rs` syntax and the gated dep
  - Enabling `vpio` on Windows/Linux is a no-op at resolution time

crates/wzp-client/src/lib.rs
  - `pub mod audio_vpio` is now #[cfg(all(feature = "vpio", target_os = "macos"))]
  - Previously `vpio` alone was enough to try to compile the Core Audio
    bindings, which would fail on non-Apple targets the moment the
    feature flag was flipped on

desktop/src-tauri/Cargo.toml
  - [target.'cfg(not(target_os = "android"))'] removed — was leaking
    vpio into Windows/Linux via the catch-all.
  - macOS: wzp-client with features = ["audio", "vpio"]
  - Windows: wzp-client with features = ["audio"]
  - Linux: wzp-client with features = ["audio"]
  - Android: wzp-client with default-features = false (unchanged)
  - Dropped the unused direct coreaudio-rs = "0.11" dep on macOS —
    wzp-desktop's own sources never call Core Audio directly.

Verified via `cargo tree --target x86_64-pc-windows-msvc -p wzp-desktop`
that the Windows target now resolves wzp-client with cpal but without
coreaudio-rs. macOS target still resolves with coreaudio (direct via
vpio feature and transitively via cpal). macOS `cargo check` still
builds cleanly.

Cross-compile from macOS hit a cargo-xwin + llvm-lib setup issue in
ring's build.rs, so the actual `cargo check --target
x86_64-pc-windows-msvc` did not complete locally. Build verification
belongs on the user's Windows x86_64 host where MSVC is present
natively.

See tasks #23 (this one), #24 (Voice Capture DSP / WASAPI Communications
for OS-level AEC on Windows), and #25 (aarch64-pc-windows-msvc support).

Cargo.lock

@@ -7202,8 +7202,9 @@ name = "wzp-desktop"
 version = "0.1.0"
 dependencies = [
  "anyhow",
- "coreaudio-rs",
+ "jni",
  "libloading 0.8.9",
+ "ndk-context",
  "rustls",
  "serde",
  "serde_json",

crates/wzp-client/Cargo.toml

@@ -23,13 +23,21 @@ serde_json = "1"
 chrono = "0.4"
 rustls = { version = "0.23", default-features = false, features = ["ring", "std"] }
 cpal = { version = "0.15", optional = true }
-coreaudio-rs = { version = "0.11", optional = true }
 libc = "0.2"
 
+# coreaudio-rs is Apple-framework-only; gate it to macOS so enabling
+# the `vpio` feature from a non-macOS target builds cleanly instead of
+# pulling in a crate that can only link against Apple frameworks.
+[target.'cfg(target_os = "macos")'.dependencies]
+coreaudio-rs = { version = "0.11", optional = true }
+
 [features]
 default = []
 audio = ["cpal"]
-vpio = ["coreaudio-rs"]
+# vpio enables coreaudio-rs but that dep is itself gated to macOS above,
+# so enabling this feature on Windows/Linux is a no-op (the audio_vpio
+# module is also #[cfg(target_os = "macos")] in lib.rs).
+vpio = ["dep:coreaudio-rs"]
 
 [[bin]]
 name = "wzp-client"

crates/wzp-client/src/lib.rs

@@ -10,7 +10,10 @@
 pub mod audio_io;
 #[cfg(feature = "audio")]
 pub mod audio_ring;
-#[cfg(feature = "vpio")]
+// VoiceProcessingIO is an Apple Core Audio API — only compile the module
+// when the `vpio` feature is on AND we're targeting macOS. Enabling the
+// feature on Windows/Linux was previously silently broken.
+#[cfg(all(feature = "vpio", target_os = "macos"))]
 pub mod audio_vpio;
 pub mod bench;
 pub mod call;

desktop/src-tauri/Cargo.toml

@@ -51,16 +51,29 @@ wzp-fec = { path = "../../crates/wzp-fec" }
 wzp-crypto = { path = "../../crates/wzp-crypto" }
 wzp-transport = { path = "../../crates/wzp-transport" }
 
-# wzp-client pulls CPAL + (on macOS) VoiceProcessingIO — desktop only.
-# Android gets an oboe/AAudio backend in Step 3 of the Tauri mobile rewrite.
-[target.'cfg(not(target_os = "android"))'.dependencies]
+# wzp-client pulls in CPAL on every desktop target and, additionally on
+# macOS, VoiceProcessingIO (coreaudio-rs behind the "vpio" feature). The
+# vpio feature MUST NOT be enabled on Windows / Linux because coreaudio-rs
+# is Apple-framework-only and will fail to build. Task #24 will add a
+# matching Windows Voice Capture DSP path behind its own feature; until
+# then, Windows desktops use plain CPAL with AEC disabled.
+
+# macOS: CPAL + VoiceProcessingIO (hardware AEC via Core Audio).
+[target.'cfg(target_os = "macos")'.dependencies]
 wzp-client = { path = "../../crates/wzp-client", features = ["audio", "vpio"] }
 
-# Step B: depend on wzp-client on Android WITHOUT the audio/vpio features,
-# so we pull in only the platform-independent modules (call, handshake,
-# audio_ring, codec bindings). Deliberately no imports in lib.rs yet — we
-# only want to verify that compiling + linking these crates on android
-# doesn't regress the working build.
+# Windows: CPAL only. Windows AEC (WASAPI Communications / MF Voice
+# Capture DSP) lands in a follow-up task. See task #24.
+[target.'cfg(target_os = "windows")'.dependencies]
+wzp-client = { path = "../../crates/wzp-client", features = ["audio"] }
+
+# Linux: same as Windows for now — plain CPAL.
+[target.'cfg(target_os = "linux")'.dependencies]
+wzp-client = { path = "../../crates/wzp-client", features = ["audio"] }
+
+# Android: no CPAL, no vpio — audio goes through the standalone wzp-native
+# cdylib that we dlopen via libloading at runtime. See the wzp_native
+# module in src/.
 [target.'cfg(target_os = "android")'.dependencies]
 wzp-client = { path = "../../crates/wzp-client", default-features = false }
 # libloading: runtime dlopen of libwzp_native.so — the standalone cdylib
@@ -75,10 +88,6 @@ libloading = "0.8"
 jni = "0.21"
 ndk-context = "0.1"
 
-# Platform-specific
-[target.'cfg(target_os = "macos")'.dependencies]
-coreaudio-rs = "0.11"
-
 [features]
 default = ["custom-protocol"]
 custom-protocol = ["tauri/custom-protocol"]