feat(persist): reload book+view on undo/redo via projectconfig hook (R-B)

BeginLoadProjectState(isUndo) requests a deferred reload the timer drains
next tick, once REAPER has restored ext-state. Hook owns undo/redo; identity
poll still owns open/tab-switch/save-as. Fixes copy-collapse over-count
(verb-aware guard) and dangling undo point on unsaved-project bank ops.

src/main.cpp

@@ -241,6 +241,56 @@ static void OnTimer()
     reasampler::bankPanelRefresh();
 }
 
+// --- projectconfig hook: reload the session on undo/redo (R-B) ---------------
+// A Ctrl-Z / Ctrl-Shift-Z rolls back / forward the "reasampler" project ext state on
+// disk but keeps the SAME project identity (ReaProject*/GUID/.rpp path), so the timer's
+// identity poll reads it as NoOp and never re-reads ext state — the in-memory book/view
+// would stay stale until close+reopen. REAPER's projectconfig extension fires
+// BeginLoadProjectState on every project-state (re)load, INCLUDING an undo/redo restore
+// (isUndo == true for both). We hook it to drive a session reload.
+//
+// TIMING (the crux): BeginLoadProjectState is documented (reaper_plugin.h ~1203) as
+// firing BEFORE any state restore. Reading GetProjExtState synchronously here would
+// return the PRE-undo value. So we do NOT read here — we raise a one-shot reload request
+// (g_session.requestReload()) that OnTimer's poll() drains on the NEXT tick, by which
+// point REAPER has finished restoring the <EXTSTATE> block and GetProjExtState returns
+// the POST-undo value. Deterministic, event-driven — NOT ext-state content polling.
+//
+// GATED ON isUndo: a normal project open also fires BeginLoadProjectState (isUndo=false);
+// we ignore that here so a normal open flows solely through the timer's identity-transition
+// Load path (no double load). Only undo/redo (isUndo=true) requests the reload.
+static void OnBeginLoadProjectState(bool isUndo, project_config_extension_t* /*reg*/)
+{
+    if (isUndo)
+        g_session.requestReload();
+}
+
+// ProcessExtensionLine / SaveExtensionConfig are intentional no-ops: ReaSampler stores
+// its state via project EXT STATE (SetProjExtState/GetProjExtState under "reasampler"),
+// which REAPER persists in its own <EXTSTATE> RPP block — NOT via this extension's own
+// project lines. We register the struct ONLY for the BeginLoadProjectState undo/redo
+// notification. Returning false from ProcessExtensionLine means "not our line" so REAPER
+// keeps dispatching (we claim none). SaveExtensionConfig writes nothing.
+static bool OnProcessExtensionLine(const char* /*line*/, ProjectStateContext* /*ctx*/,
+                                   bool /*isUndo*/, project_config_extension_t* /*reg*/)
+{
+    return false;  // we own no project lines — ext state carries our data
+}
+
+static void OnSaveExtensionConfig(ProjectStateContext* /*ctx*/, bool /*isUndo*/,
+                                  project_config_extension_t* /*reg*/)
+{
+    // Nothing to write: our data rides in ext state, not project lines.
+}
+
+// Storage must outlive registration — REAPER holds this pointer until we unregister it.
+static project_config_extension_t g_projectConfig{
+    &OnProcessExtensionLine,
+    &OnSaveExtensionConfig,
+    &OnBeginLoadProjectState,
+    nullptr,  // userData
+};
+
 // --- Scope-action source resolution -----------------------------------------
 // The three scope actions (item / track / master) each resolve to (1) an exact
 // render range in project seconds — razor-else-time, inferred here — and (2) the
@@ -786,6 +836,7 @@ extern "C" REAPER_PLUGIN_DLL_EXPORT int REAPER_PLUGIN_ENTRYPOINT(
             }
 
             g_rec->Register("-timer", (void*)&OnTimer);
+            g_rec->Register("-projectconfig", (void*)&g_projectConfig);
             g_rec->Register("-toggleaction", (void*)&OnToggleAction);
             g_rec->Register("-hookcommand", (void*)&OnHookCommand);
             // Tear down the Design View action family (D4) — mirror-unregisters each
@@ -957,6 +1008,12 @@ extern "C" REAPER_PLUGIN_DLL_EXPORT int REAPER_PLUGIN_ENTRYPOINT(
     // state, on a Save-As it relocates the bank folder under the new .rpp.
     rec->Register("timer", (void*)&OnTimer);
 
+    // Register the projectconfig hook so an UNDO/REDO state restore reloads the
+    // session's book + view from the restored ext state (R-B). The timer's identity
+    // poll cannot see an undo (same project identity), so this hook owns undo/redo; it
+    // requests a deferred reload that the next timer tick drains (see the hook comment).
+    rec->Register("projectconfig", (void*)&g_projectConfig);
+
     ShowConsoleMsg("ReaSampler loaded.\n");
 
     return 1;  // success — REAPER keeps us loaded