refactor(capture): expose offline tail as docked-panel toggle

Replace the ...-with-tail variant actions with a pure tail_control module +
a docked-panel footer toggle (Off/Auto/Manual) read by CAPTURE_ITEM/TRACK.
Default None (byte-identical); Manual fixed 2s, in-memory session lifetime.
Tail mechanism unchanged; retired the variant ids.

src/tail_control.cpp

@@ -0,0 +1,33 @@
+// tail_control — pure implementation. See tail_control.h. NO REAPER / SWELL / vendor.
+
+#include "tail_control.h"
+
+#include <algorithm>
+
+namespace reasampler {
+
+TailMode cycleTailMode(TailMode current) {
+    switch (current) {
+        case TailMode::None:   return TailMode::Auto;
+        case TailMode::Auto:   return TailMode::Manual;
+        case TailMode::Manual: return TailMode::None;
+    }
+    return TailMode::None;  // unreachable for a valid enum; fail to the safe default
+}
+
+double clampManualMs(double manualMs) {
+    // Same runaway guard the pure tailRenderSettingsFor applies to Manual: floor a
+    // negative request to 0, cap at the 8 s ceiling.
+    return std::clamp(manualMs, 0.0, kMaxTailMs);
+}
+
+std::string tailToggleLabel(const TailSetting& setting) {
+    switch (setting.mode) {
+        case TailMode::None:   return "Tail: Off";
+        case TailMode::Auto:   return "Tail: Auto";
+        case TailMode::Manual: return "Tail: Manual";
+    }
+    return "Tail: Off";  // unreachable for a valid enum; fail to the safe default
+}
+
+} // namespace reasampler