feat(audio): real noise suppression via vendored RNNoise (send + receive)

The two-sided NR plumbing (RemoteStream::recv_ns + the per-listener
vc_set_remote_stream noise_reduction toggle) was wired but inert:
ApmProcessor::create() returned a no-op passthrough, because the
originally-planned webrtc-audio-processing has no working Windows/macOS
build. Drop in RNNoise as the real backend behind the same ApmProcessor
interface, lighting up both NR paths.

- Vendor RNNoise (BSD-3 + CC0) at third_party/rnnoise/ — the vcpkg port
  is !windows !arm, so it can't cover our primary targets. Shrunk int8
  model (78MB -> 11.7MB via upstream scripts/shrink_model.sh), built as a
  standalone C static lib with no RTCD (portable scalar path on x86,
  auto-NEON on arm64) under -DDISABLE_DEBUG_FLOAT. Model is baked in
  (rnnoise_create(NULL)); no runtime file.
- New RnnoiseProcessor (core/src/audio/apm_processor.cpp) selected by
  ApmProcessor::create() when VOICECAT_HAS_NS. Mono/48kHz/480-sample;
  our clock is fixed 48kHz and Opus frame sizes are multiples of 480, so
  no resampling. RT-safe: allocates at construction, lock-free in the
  capture/playback callbacks.
- Receive-side: lit up via the factory; gated to mono streams (a stereo
  stream is a screen-audio share, not voice).
- Send-side (new): vc_set_input_noise_reduction(client, enable) ABI +
  vc_client::mic_ns_, run before input gain/VAD in on_capture_frame. A
  stereo mic is downmixed to mono ONLY when NR is on — with NR off a
  stereo mic keeps full stereo (never collapse mic quality unasked).
- Enable C as a project language for the vendored lib.
- New noise_suppression test: white noise through ApmProcessor::create()
  drops ~99.9% RMS. ctest --preset dev green, 28/28. windows-client DLL
  builds clean with vc_set_input_noise_reduction exported, system-only deps.
- Docs synced: voice.md §10, tech-stack.md §1/§5, third_party/README.md,
  vcpkg.json note, PROGRESS.md, CLAUDE.md.

Client on/off UI toggles (Windows/macOS/iOS) are the remaining follow-up.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

third_party/rnnoise/include/rnnoise.h

@@ -0,0 +1,131 @@
+/* Copyright (c) 2018 Gregor Richards
+ * Copyright (c) 2017 Mozilla */
+/*
+   Redistribution and use in source and binary forms, with or without
+   modification, are permitted provided that the following conditions
+   are met:
+
+   - Redistributions of source code must retain the above copyright
+   notice, this list of conditions and the following disclaimer.
+
+   - Redistributions in binary form must reproduce the above copyright
+   notice, this list of conditions and the following disclaimer in the
+   documentation and/or other materials provided with the distribution.
+
+   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+   ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+   A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR
+   CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+   EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+   PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+   PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+   LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+   NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+   SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*/
+
+#ifndef RNNOISE_H
+#define RNNOISE_H 1
+
+#include <stdio.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef RNNOISE_EXPORT
+# if defined(WIN32)
+#  if defined(RNNOISE_BUILD) && defined(DLL_EXPORT)
+#   define RNNOISE_EXPORT __declspec(dllexport)
+#  else
+#   define RNNOISE_EXPORT
+#  endif
+# elif defined(__GNUC__) && defined(RNNOISE_BUILD)
+#  define RNNOISE_EXPORT __attribute__ ((visibility ("default")))
+# else
+#  define RNNOISE_EXPORT
+# endif
+#endif
+
+typedef struct DenoiseState DenoiseState;
+typedef struct RNNModel RNNModel;
+
+/**
+ * Return the size of DenoiseState
+ */
+RNNOISE_EXPORT int rnnoise_get_size(void);
+
+/**
+ * Return the number of samples processed by rnnoise_process_frame at a time
+ */
+RNNOISE_EXPORT int rnnoise_get_frame_size(void);
+
+/**
+ * Initializes a pre-allocated DenoiseState
+ *
+ * If model is NULL the default model is used.
+ *
+ * See: rnnoise_create() and rnnoise_model_from_file()
+ */
+RNNOISE_EXPORT int rnnoise_init(DenoiseState *st, RNNModel *model);
+
+/**
+ * Allocate and initialize a DenoiseState
+ *
+ * If model is NULL the default model is used.
+ *
+ * The returned pointer MUST be freed with rnnoise_destroy().
+ */
+RNNOISE_EXPORT DenoiseState *rnnoise_create(RNNModel *model);
+
+/**
+ * Free a DenoiseState produced by rnnoise_create.
+ *
+ * The optional custom model must be freed by rnnoise_model_free() after.
+ */
+RNNOISE_EXPORT void rnnoise_destroy(DenoiseState *st);
+
+/**
+ * Denoise a frame of samples
+ *
+ * in and out must be at least rnnoise_get_frame_size() large.
+ */
+RNNOISE_EXPORT float rnnoise_process_frame(DenoiseState *st, float *out, const float *in);
+
+/**
+ * Load a model from a memory buffer
+ *
+ * It must be deallocated with rnnoise_model_free() and the buffer must remain
+ * valid until after the returned object is destroyed.
+ */
+RNNOISE_EXPORT RNNModel *rnnoise_model_from_buffer(const void *ptr, int len);
+
+
+/**
+ * Load a model from a file
+ *
+ * It must be deallocated with rnnoise_model_free() and the file must not be
+ * closed until the returned object is destroyed.
+ */
+RNNOISE_EXPORT RNNModel *rnnoise_model_from_file(FILE *f);
+
+/**
+ * Load a model from a file name
+ *
+ * It must be deallocated with rnnoise_model_free()
+ */
+RNNOISE_EXPORT RNNModel *rnnoise_model_from_filename(const char *filename);
+
+/**
+ * Free a custom model
+ *
+ * It must be called after all the DenoiseStates referring to it are freed.
+ */
+RNNOISE_EXPORT void rnnoise_model_free(RNNModel *model);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif