T2: realtime capture tail trimming

Auto/Manual/Off tail handling for realtime track-tap capture. Auto scans
recorded PCM backward for last frame above -72 dB and truncates the wav
header-aware (8 s cap); Manual pads a fixed tail; Off is byte-identical.
wav_trim rejects WAVE_FORMAT_EXTENSIBLE with non-float SubFormat GUID.

src/peaks.cpp

@@ -2,6 +2,7 @@
 
 #include <algorithm>
 #include <climits>
+#include <cmath>
 
 // peaks implementation.
 //
@@ -63,4 +64,31 @@ Envelope computeEnvelope(const std::vector<AudioSample>& interleaved,
     return envelope;
 }
 
+std::size_t lastFrameAboveThreshold(const std::vector<AudioSample>& interleaved,
+                                    std::size_t channelCount,
+                                    std::size_t frameCount,
+                                    AudioSample linearThreshold) {
+    if (channelCount == 0) return kNoFrameAboveThreshold;
+
+    // Clamp to what the buffer actually holds — a caller frameCount that overstates
+    // the buffer must never read past the end (mirror of computeEnvelope's guard).
+    const std::size_t availableFrames = interleaved.size() / channelCount;
+    const std::size_t frames = std::min(frameCount, availableFrames);
+    if (frames == 0) return kNoFrameAboveThreshold;
+
+    // Scan backward: the first frame (from the end) whose loudest channel exceeds the
+    // threshold is the last audible frame. `f` runs frames..1 so `f-1` never wraps.
+    for (std::size_t f = frames; f > 0; --f) {
+        const std::size_t frame = f - 1;
+        const std::size_t base = frame * channelCount;
+        AudioSample peak = 0.0f;
+        for (std::size_t c = 0; c < channelCount; ++c) {
+            const AudioSample a = std::fabs(interleaved[base + c]);
+            peak = std::max(peak, a);
+        }
+        if (peak > linearThreshold) return frame;
+    }
+    return kNoFrameAboveThreshold;
+}
+
 } // namespace reasampler