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/render_settings.cpp

@@ -56,6 +56,24 @@ TailRenderSettings tailRenderSettingsFor(TailMode mode, double manualTailMs) {
     return t;
 }
 
+double realtimeRecordWindowEnd(TailMode mode, double rangeEndSeconds,
+                               double manualTailMs) {
+    switch (mode) {
+        case TailMode::None:
+            // Exact — no extra recording (byte-identical to today's realtime capture).
+            return rangeEndSeconds;
+        case TailMode::Auto:
+            // The 8 s runaway cap past the range end; the decay-trim shortens it later.
+            return rangeEndSeconds + kMaxTailSeconds;
+        case TailMode::Manual:
+            // Fixed window: range + the set length, clamped to the 8 s cap (the same
+            // runaway guard the offline Manual path applies). Negative floors to 0.
+            return rangeEndSeconds + std::clamp(manualTailMs, 0.0, kMaxTailMs) / 1000.0;
+    }
+    // Unreachable for a valid enum; fail closed to exact bounds (never a stray tail).
+    return rangeEndSeconds;
+}
+
 RenderSettingsChoice renderSettingsFor(SourceMode mode, double /*wetDry*/) {
     // `wetDry` is accepted so CaptureRequest.wetDry remains the seam for future
     // dry work (M10 null test), but it does not affect this mapping. FX scoping is