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.h

@@ -0,0 +1,49 @@
+#pragma once
+// tail_control — the REAPER-free logic behind the docked bank_panel's tail-mode
+// toggle. The panel shell (bank_panel.cpp) owns the SWELL window, LICE drawing, and
+// click hit-testing; what is NOT DAW-bound — the cycle order, the manual-length
+// clamp, and the toggle's label text — lives here so it is unit-tested outside the
+// DAW (CLAUDE.md §load-bearing split). Mirror of bank_grid / mode_switch.
+//
+// PURE MODULE: NO REAPER types, NO SWELL, NO vendor/ includes. Standard library
+// only (plus render_settings for the pure TailMode enum). Builds and unit-tests
+// without REAPER.
+
+#include <string>
+
+#include "render_settings.h"  // TailMode (pure enum) — the three-state tail contract
+
+namespace reasampler {
+
+// The Manual-mode starting length. 2 s is a musically useful default tail (a bar of
+// reverb throw at a moderate tempo) that is well under the 8 s cap. A fine-adjust UI
+// (+/- click zones or scroll) is a noted follow-on; this pass ships a fixed default.
+inline constexpr double kDefaultManualTailMs = 2000.0;
+
+// The panel's current tail setting: the mode plus the length used ONLY when the
+// mode is Manual. Held as in-memory panel/session state (bank_panel.cpp), default
+// None so a capture with no explicit choice stays exact-bounds / byte-identical to
+// today. `manualMs` is a stored default a future fine-adjust UI can tune; it is
+// clamped to the 8 s cap (kMaxTailMs) before it ever reaches a CaptureRequest.
+struct TailSetting {
+    TailMode mode     = TailMode::None;
+    double   manualMs = kDefaultManualTailMs;
+};
+
+// Cycles the tail mode: None -> Auto -> Manual -> None. Pure so the wrap order is
+// pinned by a test and the panel's click handler owns no enum arithmetic of its own.
+// An out-of-range value (unreachable for a valid enum) cycles back to None.
+TailMode cycleTailMode(TailMode current);
+
+// The effective manual length a Manual capture uses: `manualMs` clamped to
+// [0, kMaxTailMs] (the runaway guard the pure tailRenderSettingsFor also applies).
+// Exposed so the panel can show the clamped value and main.cpp hands a pre-clamped
+// tailMs into the CaptureRequest. Meaningful only for TailMode::Manual.
+double clampManualMs(double manualMs);
+
+// The toggle's label for a setting, e.g. "Tail: Off", "Tail: Auto", "Tail: Manual".
+// (Manual omits the length here — the panel is unobtrusive; a length readout can be
+// added with the fine-adjust follow-on.) Pure so the exact strings are test-pinned.
+std::string tailToggleLabel(const TailSetting& setting);
+
+} // namespace reasampler