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.h

@@ -114,6 +114,20 @@ struct TailRenderSettings {
 // the Auto default or an explicit request (spec §Manual override). Pure + tested.
 TailRenderSettings tailRenderSettingsFor(TailMode mode, double manualTailMs);
 
+// The REALTIME record-window end (in project seconds) a tail mode records to, given
+// the request's exact range end (docs/product/capture-tail.md §The realtime path).
+// Realtime does NOT drive RENDER_*; it records a generous window and trims later, so
+// the window end is where the transport actually stops:
+//   None   -> rangeEndSeconds                 (exact — no extra recording).
+//   Auto   -> rangeEndSeconds + kMaxTailSeconds (the 8 s runaway cap; trimmed later).
+//   Manual -> rangeEndSeconds + clamp(manualTailMs, kMaxTailMs)/1000 (fixed, no trim).
+// `manualTailMs` is used ONLY for Manual. Pure so the mode->window arithmetic (and
+// the Manual clamp) is unit-tested outside the DAW; the backend applies the returned
+// end to the record time selection. Shared -72 dB / 8 s constants are the same ones
+// the offline tail uses (single source of truth).
+double realtimeRecordWindowEnd(TailMode mode, double rangeEndSeconds,
+                               double manualTailMs);
+
 // The RENDER_SETTINGS value for a given source mode. `supported` is false only
 // for SourceMode::Realtime (that is the M8 backend, not offline render).
 struct RenderSettingsChoice {