Add tail manual fine-adjust (scroll) + per-project tail persistence

Relocate TailSetting into ReaSamplerSession; persist serializes it to
a new forever-stable "tail_setting" ext-state key (load-per-project,
save-on-save). Footer scroll-wheel adjusts Manual length in 250ms
steps; label now shows "Tail: Manual X.Xs". Pure step/label/JSON
round-trip covered by tests.

tests/test_tail_control.cpp

@@ -49,15 +49,53 @@ static void testManualClampCapsAtEightSeconds() {
     CHECK(clampManualMs(-100.0) == 0.0);
 }
 
+// --- adjustManualMs: the scroll-wheel fine-adjust arithmetic ------------------
+
+static void testAdjustUpAndDownBySteps() {
+    // Positive notches lengthen, negative shorten, in whole kManualStepMs increments.
+    CHECK(adjustManualMs(2000.0, 1, kManualStepMs)  == 2250.0);
+    CHECK(adjustManualMs(2000.0, -1, kManualStepMs) == 1750.0);
+    CHECK(adjustManualMs(2000.0, 4, kManualStepMs)  == 3000.0);  // 4 * 250
+    CHECK(adjustManualMs(2000.0, 0, kManualStepMs)  == 2000.0);  // no notch, no move
+}
+
+static void testAdjustClampsAtUpperBound() {
+    // Scrolling up past the 8 s cap saturates AT the cap, never beyond.
+    CHECK(adjustManualMs(kMaxTailMs, 1, kManualStepMs)         == kMaxTailMs);
+    CHECK(adjustManualMs(kMaxTailMs - 100.0, 10, kManualStepMs) == kMaxTailMs);
+}
+
+static void testAdjustClampsAtLowerBound() {
+    // Scrolling down past 0 floors at 0, never negative.
+    CHECK(adjustManualMs(0.0, -1, kManualStepMs)    == 0.0);
+    CHECK(adjustManualMs(100.0, -10, kManualStepMs) == 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;
+    // Off/Auto carry NO length regardless of manualMs.
+    off.manualMs   = 5000.0;
+    autoM.manualMs = 5000.0;
     CHECK(tailToggleLabel(off)   == "Tail: Off");
     CHECK(tailToggleLabel(autoM) == "Tail: Auto");
-    CHECK(tailToggleLabel(man)   == "Tail: Manual");
+}
+
+static void testManualLabelRendersLengthInSeconds() {
+    // Manual appends the length in seconds to one decimal — pin the format and the
+    // boundary values (0.0s, the 2 s default, the 8 s cap).
+    TailSetting man; man.mode = TailMode::Manual;
+    man.manualMs = 0.0;
+    CHECK(tailToggleLabel(man) == "Tail: Manual 0.0s");
+    man.manualMs = kDefaultManualTailMs;  // 2000 ms
+    CHECK(tailToggleLabel(man) == "Tail: Manual 2.0s");
+    man.manualMs = kMaxTailMs;            // 8000 ms
+    CHECK(tailToggleLabel(man) == "Tail: Manual 8.0s");
+    // An over-cap stored value renders at the CLAMPED length, never past the cap.
+    man.manualMs = kMaxTailMs + 3000.0;
+    CHECK(tailToggleLabel(man) == "Tail: Manual 8.0s");
 }
 
 static void testDefaultSettingIsOff() {
@@ -69,13 +107,65 @@ static void testDefaultSettingIsOff() {
     CHECK(tailToggleLabel(s) == "Tail: Off");
 }
 
+// --- serialize/deserialize: per-project persistence round-trip ----------------
+
+static bool settingsEqual(const TailSetting& a, const TailSetting& b) {
+    return a.mode == b.mode && a.manualMs == b.manualMs;
+}
+
+static void testRoundTripNoneDefault() {
+    TailSetting s;  // None + 2 s default
+    auto back = deserializeTailSetting(serializeTailSetting(s));
+    CHECK(back.has_value());
+    CHECK(back && settingsEqual(*back, s));
+}
+
+static void testRoundTripManualArbitraryMs() {
+    // A non-round manual length must round-trip bit-for-bit (17-sig-digit emit).
+    TailSetting s; s.mode = TailMode::Manual; s.manualMs = 3141.592653589793;
+    auto back = deserializeTailSetting(serializeTailSetting(s));
+    CHECK(back.has_value());
+    CHECK(back && settingsEqual(*back, s));
+}
+
+static void testRoundTripAuto() {
+    TailSetting s; s.mode = TailMode::Auto; s.manualMs = 500.0;
+    auto back = deserializeTailSetting(serializeTailSetting(s));
+    CHECK(back.has_value());
+    CHECK(back && settingsEqual(*back, s));
+}
+
+static void testDeserializeEmptyIsDefault() {
+    // An absent/empty stored value (older project) -> nullopt, so the caller falls
+    // back to the default. This is the graceful-old-project path the brief requires.
+    CHECK(!deserializeTailSetting("").has_value());
+}
+
+static void testDeserializeMalformedIsDefault() {
+    // Garbage, a missing key, or an unknown mode enumerant -> nullopt (no crash).
+    CHECK(!deserializeTailSetting("not json at all").has_value());
+    CHECK(!deserializeTailSetting("{\"mode\":1}").has_value());        // manualMs missing
+    CHECK(!deserializeTailSetting("{\"manualMs\":2000}").has_value()); // mode missing
+    CHECK(!deserializeTailSetting("{\"mode\":9,\"manualMs\":2000}").has_value()); // bad enum
+    CHECK(!deserializeTailSetting("{\"mode\":x,\"manualMs\":2000}").has_value()); // non-numeric
+}
+
 int main() {
     testCycleOrderIsNoneAutoManualNone();
     testCycleThreeStepsReturnsToStart();
     testManualClampInRangeIsUnchanged();
     testManualClampCapsAtEightSeconds();
+    testAdjustUpAndDownBySteps();
+    testAdjustClampsAtUpperBound();
+    testAdjustClampsAtLowerBound();
     testLabelStringsPerMode();
+    testManualLabelRendersLengthInSeconds();
     testDefaultSettingIsOff();
+    testRoundTripNoneDefault();
+    testRoundTripManualArbitraryMs();
+    testRoundTripAuto();
+    testDeserializeEmptyIsDefault();
+    testDeserializeMalformedIsDefault();
 
     if (g_fail == 0) std::printf("tail_control: all tests passed\n");
     else             std::printf("tail_control: %d CHECK(s) FAILED\n", g_fail);