feat(capture): offline auto/manual tail via pure TailMode mapping

Add TailMode{None,Auto,Manual} + derived -72dB TRIMEND + 8s cap in render_settings;
wire into OfflineRenderBackend with snapshot/restore of the tail settings; add
CAPTURE_ITEM_TAIL/CAPTURE_TRACK_TAIL variant actions (plain actions stay exact-bounds).

src/capture.h

@@ -20,6 +20,7 @@
 #include <vector>
 
 #include "bank_model.h"
+#include "render_settings.h"  // TailMode (pure) — the three-state tail contract
 
 // MediaTrack is forward-declared (like track_guid.h) so this header stays
 // REAPER-free while RealtimeRecordBackend::begin can take the resolved source
@@ -64,10 +65,13 @@ struct CaptureRequest {
     // it stays source-agnostic, driven entirely by the request).
     std::vector<std::string> trackGuids;
 
-    // Render tail. Default OFF for the spike (exact bounds, no added silence —
-    // precision invariant). M7 makes this bindable.
-    bool   renderTail   = false;
-    double tailMs       = 0.0;
+    // Render tail (docs/product/capture-tail.md §The three tail states). Default
+    // None: exact bounds, no added silence — the precision invariant, and the only
+    // mode valid for null-test / verify captures. `tailMs` is meaningful ONLY for
+    // TailMode::Manual (clamped to the 8 s cap by the pure mapping); Auto uses the
+    // 8 s cap + -72 dB trim internally, None ignores it.
+    TailMode tailMode = TailMode::None;
+    double   tailMs   = 0.0;
 
     // Output format. 0 sampleRate => follow project rate (deterministic: the
     // project rate is fixed for a given project).