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.

tests/test_tail_control.cpp

@@ -0,0 +1,83 @@
+// Standalone tests for reasampler::tail_control — no REAPER, no framework. Covers
+// the pure pieces behind the docked panel's tail-mode toggle: the cycle order
+// (None -> Auto -> Manual -> None), the manual-length clamp to the 8 s cap, and the
+// toggle label text. The drawing / click hit-testing is DAW-verified in bank_panel.
+
+#include "../src/tail_control.h"
+
+#include <cstdio>
+#include <string>
+
+using namespace reasampler;
+
+static int g_fail = 0;
+#define CHECK(cond) do { if(!(cond)) { \
+    std::printf("FAIL line %d: %s\n", __LINE__, #cond); ++g_fail; } } while(0)
+
+// --- cycleTailMode: the toggle order ------------------------------------------
+
+static void testCycleOrderIsNoneAutoManualNone() {
+    // None -> Auto -> Manual -> None, wrapping. The click handler relies on exactly
+    // this order; a reorder (e.g. skipping Manual) would fail here.
+    CHECK(cycleTailMode(TailMode::None)   == TailMode::Auto);
+    CHECK(cycleTailMode(TailMode::Auto)   == TailMode::Manual);
+    CHECK(cycleTailMode(TailMode::Manual) == TailMode::None);
+}
+
+static void testCycleThreeStepsReturnsToStart() {
+    // Three cycles from any state land back on that state (a full lap of the 3-cycle).
+    TailMode m = TailMode::None;
+    m = cycleTailMode(m);
+    m = cycleTailMode(m);
+    m = cycleTailMode(m);
+    CHECK(m == TailMode::None);
+}
+
+// --- clampManualMs: the runaway guard -----------------------------------------
+
+static void testManualClampInRangeIsUnchanged() {
+    // A value inside [0, kMaxTailMs] passes through untouched.
+    CHECK(clampManualMs(kDefaultManualTailMs) == kDefaultManualTailMs);
+    CHECK(clampManualMs(0.0) == 0.0);
+    CHECK(clampManualMs(kMaxTailMs) == kMaxTailMs);
+}
+
+static void testManualClampCapsAtEightSeconds() {
+    // Over the 8 s cap clamps to kMaxTailMs; negative floors to 0. This mirrors the
+    // clamp in tailRenderSettingsFor, so the panel and the render agree on the bound.
+    CHECK(clampManualMs(kMaxTailMs + 5000.0) == kMaxTailMs);
+    CHECK(clampManualMs(-100.0) == 0.0);
+}
+
+// --- tailToggleLabel: the exact strings the panel draws -----------------------
+
+static void testLabelStringsPerMode() {
+    TailSetting off;   off.mode   = TailMode::None;
+    TailSetting autoM; autoM.mode = TailMode::Auto;
+    TailSetting man;   man.mode   = TailMode::Manual;
+    CHECK(tailToggleLabel(off)   == "Tail: Off");
+    CHECK(tailToggleLabel(autoM) == "Tail: Auto");
+    CHECK(tailToggleLabel(man)   == "Tail: Manual");
+}
+
+static void testDefaultSettingIsOff() {
+    // The zero-value setting is None (Off) with the 2 s manual default — the safe
+    // default the panel starts in so captures stay exact-bounds until opt-in.
+    TailSetting s;
+    CHECK(s.mode == TailMode::None);
+    CHECK(s.manualMs == kDefaultManualTailMs);
+    CHECK(tailToggleLabel(s) == "Tail: Off");
+}
+
+int main() {
+    testCycleOrderIsNoneAutoManualNone();
+    testCycleThreeStepsReturnsToStart();
+    testManualClampInRangeIsUnchanged();
+    testManualClampCapsAtEightSeconds();
+    testLabelStringsPerMode();
+    testDefaultSettingIsOff();
+
+    if (g_fail == 0) std::printf("tail_control: all tests passed\n");
+    else             std::printf("tail_control: %d CHECK(s) FAILED\n", g_fail);
+    return g_fail == 0 ? 0 : 1;
+}