feat(audio): Implement mock audio backend for testing (Task #51.2)

Created MockAudioBackend for deterministic audio event recording and
timing verification. Enables robust tracker synchronization testing.

Changes:
- Created VoiceTriggerEvent structure (timestamp, spec_id, volume, pan)
- Implemented MockAudioBackend with event recording capabilities
- Added time tracking: manual (advance_time) and automatic (on_frames_rendered)
- Created test_mock_backend.cc with 6 comprehensive test scenarios
- Verified synth integration and audio_render_silent compatibility
- Added to CMake test builds

All test infrastructure guarded by #if !defined(STRIP_ALL). Zero size
impact on production build. All 14 tests pass.

handoff(Claude): Task #51.2 complete, mock backend ready for tracker tests

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

1 files changed, 215 insertions, 0 deletions

diff --git a/src/tests/test_mock_backend.cc b/src/tests/test_mock_backend.cc
new file mode 100644
index 0000000..9173bb2
--- /dev/null
+++ b/src/tests/test_mock_backend.cc
@@ -0,0 +1,215 @@
+// This file is part of the 64k demo project.
+// It tests the MockAudioBackend implementation.
+// Verifies event recording, time tracking, and synth integration.
+
+#include "audio/mock_audio_backend.h"
+#include "audio/audio.h"
+#include "audio/synth.h"
+#include <assert.h>
+#include <stdio.h>
+#include <cmath>
+
+#if !defined(STRIP_ALL)
+
+void test_event_recording() {
+  MockAudioBackend backend;
+
+  // Initially no events
+  assert(backend.get_events().size() == 0);
+  assert(backend.get_current_time() == 0.0f);
+
+  // Simulate voice trigger
+  backend.on_voice_triggered(0.5f, 3, 0.75f, -0.25f);
+
+  // Verify event recorded
+  const auto& events = backend.get_events();
+  assert(events.size() == 1);
+  assert(events[0].timestamp_sec == 0.5f);
+  assert(events[0].spectrogram_id == 3);
+  assert(events[0].volume == 0.75f);
+  assert(events[0].pan == -0.25f);
+
+  // Record multiple events
+  backend.on_voice_triggered(1.0f, 5, 1.0f, 0.0f);
+  backend.on_voice_triggered(1.5f, 3, 0.5f, 0.5f);
+
+  assert(backend.get_events().size() == 3);
+  assert(events[1].timestamp_sec == 1.0f);
+  assert(events[2].timestamp_sec == 1.5f);
+
+  // Clear events
+  backend.clear_events();
+  assert(backend.get_events().size() == 0);
+
+  printf("Event recording test PASSED\n");
+}
+
+void test_time_tracking() {
+  MockAudioBackend backend;
+
+  // Test manual time advance
+  assert(backend.get_current_time() == 0.0f);
+
+  backend.advance_time(0.5f);
+  assert(backend.get_current_time() == 0.5f);
+
+  backend.advance_time(1.0f);
+  assert(backend.get_current_time() == 1.5f);
+
+  // Test time setting
+  backend.set_time(10.0f);
+  assert(backend.get_current_time() == 10.0f);
+
+  printf("Time tracking test PASSED\n");
+}
+
+void test_frame_rendering() {
+  MockAudioBackend backend;
+
+  // Simulate frame rendering (32000 Hz sample rate)
+  // 1 second = 32000 frames
+  backend.on_frames_rendered(16000); // 0.5 seconds
+  assert(std::abs(backend.get_current_time() - 0.5f) < 0.001f);
+
+  backend.on_frames_rendered(16000); // Another 0.5 seconds
+  assert(std::abs(backend.get_current_time() - 1.0f) < 0.001f);
+
+  backend.on_frames_rendered(32000); // 1 second
+  assert(std::abs(backend.get_current_time() - 2.0f) < 0.001f);
+
+  printf("Frame rendering test PASSED\n");
+}
+
+void test_synth_integration() {
+  MockAudioBackend backend;
+  audio_set_backend(&backend);
+
+  synth_init();
+
+  // Create dummy spectrogram
+  float data[DCT_SIZE * 10] = {0};
+  data[0] = 100.0f; // DC component
+
+  Spectrogram spec = {data, data, 10};
+  int spec_id = synth_register_spectrogram(&spec);
+  assert(spec_id >= 0);
+
+  // Trigger voice - should be recorded at time 0
+  synth_trigger_voice(spec_id, 0.8f, -0.3f);
+
+  // Verify event recorded
+  const auto& events = backend.get_events();
+  assert(events.size() == 1);
+  assert(events[0].timestamp_sec == 0.0f); // Before any rendering
+  assert(events[0].spectrogram_id == spec_id);
+  assert(events[0].volume == 0.8f);
+  assert(events[0].pan == -0.3f);
+
+  // Render some frames to advance time
+  float output[1024] = {0};
+  synth_render(output, 512); // ~0.016 sec at 32kHz
+
+  // Verify synth updated its time
+  // (Note: synth time is internal, mock doesn't track it from render)
+
+  // Trigger another voice after rendering
+  synth_trigger_voice(spec_id, 1.0f, 0.5f);
+
+  assert(events.size() == 2);
+  // Second trigger should have timestamp > 0
+  assert(events[1].timestamp_sec > 0.0f);
+  assert(events[1].timestamp_sec < 0.02f); // ~512 frames = ~0.016 sec
+
+  printf("Synth integration test PASSED\n");
+}
+
+void test_multiple_voices() {
+  MockAudioBackend backend;
+  audio_set_backend(&backend);
+
+  synth_init();
+
+  // Create multiple spectrograms
+  float data1[DCT_SIZE * 5] = {0};
+  float data2[DCT_SIZE * 5] = {0};
+  float data3[DCT_SIZE * 5] = {0};
+
+  Spectrogram spec1 = {data1, data1, 5};
+  Spectrogram spec2 = {data2, data2, 5};
+  Spectrogram spec3 = {data3, data3, 5};
+
+  int id1 = synth_register_spectrogram(&spec1);
+  int id2 = synth_register_spectrogram(&spec2);
+  int id3 = synth_register_spectrogram(&spec3);
+
+  // Trigger multiple voices at once
+  synth_trigger_voice(id1, 1.0f, -1.0f);
+  synth_trigger_voice(id2, 0.5f, 0.0f);
+  synth_trigger_voice(id3, 0.75f, 1.0f);
+
+  // Verify all recorded
+  const auto& events = backend.get_events();
+  assert(events.size() == 3);
+
+  // Verify each has correct properties
+  assert(events[0].spectrogram_id == id1);
+  assert(events[1].spectrogram_id == id2);
+  assert(events[2].spectrogram_id == id3);
+
+  assert(events[0].volume == 1.0f);
+  assert(events[1].volume == 0.5f);
+  assert(events[2].volume == 0.75f);
+
+  assert(events[0].pan == -1.0f);
+  assert(events[1].pan == 0.0f);
+  assert(events[2].pan == 1.0f);
+
+  printf("Multiple voices test PASSED\n");
+}
+
+void test_audio_render_silent_integration() {
+  MockAudioBackend backend;
+  audio_set_backend(&backend);
+
+  audio_init();
+  synth_init();
+
+  // Create a spectrogram
+  float data[DCT_SIZE * 5] = {0};
+  Spectrogram spec = {data, data, 5};
+  int spec_id = synth_register_spectrogram(&spec);
+
+  // Trigger at t=0
+  synth_trigger_voice(spec_id, 1.0f, 0.0f);
+
+  // Simulate 2 seconds of silent rendering (seek/fast-forward)
+  audio_render_silent(2.0f);
+
+  // Verify backend time advanced via on_frames_rendered
+  const float expected_time = 2.0f;
+  const float actual_time = backend.get_current_time();
+  assert(std::abs(actual_time - expected_time) < 0.01f); // 10ms tolerance
+
+  audio_shutdown();
+
+  printf("audio_render_silent integration test PASSED\n");
+}
+
+#endif /* !defined(STRIP_ALL) */
+
+int main() {
+#if !defined(STRIP_ALL)
+  printf("Running MockAudioBackend tests...\n");
+  test_event_recording();
+  test_time_tracking();
+  test_frame_rendering();
+  test_synth_integration();
+  test_multiple_voices();
+  test_audio_render_silent_integration();
+  printf("All MockAudioBackend tests PASSED\n");
+  return 0;
+#else
+  printf("MockAudioBackend tests skipped (STRIP_ALL enabled)\n");
+  return 0;
+#endif /* !defined(STRIP_ALL) */
+}