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.h

@@ -20,6 +20,7 @@
 #include <string>
 
 #include "bank_model.h"
+#include "tail_control.h"
 #include "view_mode_model.h"
 
 namespace reasampler {
@@ -38,6 +39,13 @@ inline constexpr const char* kProjExtIndexKey = "bank_index";
 // FOREVER-STABLE: changing it orphans every already-saved project's view state.
 inline constexpr const char* kProjExtViewKey = "view_state";
 
+// The ext-state key the docked panel's TailSetting JSON (mode + manualMs) is stored
+// under, so the tail choice travels inside the .rpp and loads per project. Distinct
+// from the index/view keys — one namespace, three keys. FOREVER-STABLE: changing it
+// orphans every already-saved project's tail setting (which then falls back to the
+// default — graceful, but the user's saved choice would be lost).
+inline constexpr const char* kProjExtTailKey = "tail_setting";
+
 // The ext-state key holding a GUID we mint per project to establish CONTENT-BASED
 // project identity (REAPER exposes no stable per-project GUID). poll() uses it to
 // tell a genuine Save-As (same GUID, new .rpp path) apart from a project switch
@@ -80,6 +88,15 @@ public:
     ViewModeModel& view() { return view_; }
     const ViewModeModel& view() const { return view_; }
 
+    // The docked panel's tail setting (mode + manualMs), authoritative here — NOT in
+    // panel state — so it travels inside the .rpp: persist serializes it on save and
+    // replaces it on project load exactly as it treats the bank and view model. The
+    // panel reads/writes it through this seam (bank_panel holds the session), and the
+    // capture actions read it via bankPanelTailSetting. Default None / 2 s manual for
+    // an unsaved or pre-feature project (no stored key -> this default survives load).
+    TailSetting& tail() { return tail_; }
+    const TailSetting& tail() const { return tail_; }
+
     // Serialize the current bank to the active project's ext state (namespace
     // "reasampler"). Non-destructive beyond writing our own ext-state key. Safe
     // to call when there is no active/saved project (it no-ops).
@@ -110,6 +127,11 @@ private:
     // view_state (older project), so an absent key is graceful, not a crash.
     ViewModeModel view_;
 
+    // The tail setting. Default None / kDefaultManualTailMs; loadFromProject resets it
+    // to this default when a project has no stored tail_setting key (older / never-
+    // adjusted project), so an absent key is graceful. Peer to bank_/view_.
+    TailSetting tail_;
+
     // The project identity last observed by poll(), used to detect load/Save-As.
     // The GUID is the PRIMARY signal (a different stored GUID = a different project
     // of record = Load, immune to pointer recycling). The pointer disambiguates the