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",