feat(linux): WebRTC AEC3 capture/playback backend with render-side tee

Adds gold-standard Linux echo cancellation: in-app WebRTC AEC3 (Audio
Processing Module) via the webrtc-audio-processing crate, using the
same algorithm as Chrome WebRTC, Zoom, Teams, and Jitsi. Runs entirely
in-process, so it works identically on ALSA / PulseAudio / PipeWire
systems — no dependency on user-configured echo-cancel modules.

Architecture:
- New crates/wzp-client/src/audio_linux_aec.rs module (~470 lines).
  Contains LinuxAecCapture and LinuxAecPlayback, both using CPAL
  under the hood but routing samples through a shared
  Arc<webrtc_audio_processing::Processor>. The playback path tees
  each 20 ms frame into APM.process_render_frame as the echo
  reference BEFORE handing the samples to CPAL's output callback.
  The capture path runs APM.process_capture_frame on each mic frame
  in place before pushing to the audio ring buffer. This is the
  "tee the playback ring" approach that Zoom/Teams/Jitsi use.
- New `linux-aec` feature in wzp-client pulling in the
  webrtc-audio-processing crate at v2.x with the `bundled`
  sub-feature. Bundled means the vendored PulseAudio WebRTC C++
  sources are statically compiled via meson+ninja at cargo build
  time — no runtime .so dependency, avoids Debian Bookworm's stale
  libwebrtc-audio-processing-dev 0.3 package (which predates AEC3).
  Dep is target-gated to Linux, so enabling the feature on non-Linux
  is a no-op.
- lib.rs re-exports LinuxAecCapture/LinuxAecPlayback as
  AudioCapture/AudioPlayback when `linux-aec` is on, otherwise
  falls back to the CPAL audio_io path. Shared public API
  (start/ring/stop/Drop) means downstream code is unchanged.
- New `linux-aec` feature in wzp-desktop forwards to
  wzp-client/linux-aec so `cargo tauri build -- --features
  wzp-desktop/linux-aec` builds the AEC variant.

APM configuration:
- EchoCancellation: High suppression, delay-agnostic mode on,
  extended filter on, stream_delay_ms=60 initial hint
- NoiseSuppression: High
- HighPassFilter: on
- AGC: off (can fight Opus encoder's own gain staging + adaptive
  quality controller; add later if users report low mic level)

Frame size handling:
- Pipeline uses 20 ms frames (960 samples @ 48 kHz mono)
- APM requires strict 10 ms (480 samples) per call
- Each 20 ms frame is split into two 480-sample halves, APM called
  twice, halves stitched back
- Same pattern for render and capture sides
- Carry-buffer logic handles the case where CPAL delivers samples in
  arbitrary chunk sizes that don't divide 960

Build infrastructure:
- scripts/Dockerfile.linux-desktop-builder adds meson, ninja-build,
  python3, clang for the webrtc-audio-processing bundled build
- scripts/build-linux-desktop-docker.sh takes a new --aec flag that
  enables the linux-aec feature and renames the output artifacts
  with an `-aec` suffix so noAEC and AEC variants can coexist on disk

Task #30.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/Dockerfile.linux-desktop-builder

@@ -33,7 +33,22 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
         librsvg2-dev \
         libglib2.0-dev \
         patchelf \
+        meson \
+        ninja-build \
+        python3 \
+        clang \
     && rm -rf /var/lib/apt/lists/*
 
+# ── webrtc-audio-processing build requirements ──────────────────────────────
+# The `webrtc-audio-processing` Rust crate with the `bundled` feature vendors
+# the PulseAudio webrtc-audio-processing C++ library and builds it via meson
+# + ninja at `cargo build` time. That avoids Debian Bookworm's stale
+# libwebrtc-audio-processing-dev 0.3-1 package (which predates AEC3) and gives
+# us a self-contained static link — no runtime .so dependency, same algorithm
+# on every Linux distro regardless of what apt ships.
+#
+# apt deps for the bundled build: meson, ninja-build, python3, clang,
+# build-essential (already present via android-builder base).
+
 USER builder
 WORKDIR /build/source