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.

src/persist.cpp

@@ -184,6 +184,13 @@ void ReaSamplerSession::saveToActiveProject() {
     SetProjExtState(static_cast<ReaProject*>(proj), kProjExtNamespace,
                     kProjExtViewKey, viewJson.c_str());
 
+    // Additive: the docked panel's tail setting rides alongside in its own key, so the
+    // tail choice travels inside the .rpp. Independent write — does not disturb the
+    // bank_index or view_state above.
+    const std::string tailJson = serializeTailSetting(tail_);
+    SetProjExtState(static_cast<ReaProject*>(proj), kProjExtNamespace,
+                    kProjExtTailKey, tailJson.c_str());
+
     MarkProjectDirty(static_cast<ReaProject*>(proj));
 }
 
@@ -208,6 +215,23 @@ ViewModeModel loadViewModel(ReaProject* proj) {
     return std::move(*loaded);
 }
 
+// Load the tail setting from a project's tail_setting key, or return the default. An
+// absent/empty key (older / never-adjusted project) yields the default setting (None /
+// 2 s manual) — graceful, never a crash. Malformed JSON is warned and also falls back
+// to default, mirroring the bank's and view's malformed handling.
+TailSetting loadTailSetting(ReaProject* proj) {
+    if (!proj) return TailSetting{};
+    const std::string tailJson =
+        getProjExtStateString(proj, kProjExtNamespace, kProjExtTailKey);
+    if (tailJson.empty()) return TailSetting{};  // no stored setting -> default
+    std::optional<TailSetting> loaded = deserializeTailSetting(tailJson);
+    if (!loaded) {
+        ShowConsoleMsg("ReaSampler: stored tail setting is malformed — ignoring.\n");
+        return TailSetting{};
+    }
+    return *loaded;
+}
+
 }  // namespace
 
 void ReaSamplerSession::loadFromProject(void* proj, const std::string& projectDir) {
@@ -225,6 +249,11 @@ void ReaSamplerSession::loadFromProject(void* proj, const std::string& projectDi
     // only — no visibility/processing is applied here (that is D4).
     view_ = loadViewModel(static_cast<ReaProject*>(proj));
 
+    // The tail setting is restored on EVERY load path too (peer-symmetry): switching
+    // to a project with no stored setting must fall back to the default, not inherit
+    // the previous project's choice (this REPLACES the old session-carry behavior).
+    tail_ = loadTailSetting(static_cast<ReaProject*>(proj));
+
     if (!proj) {
         bank_ = BankIndex{};
         return;