feat(version): single-source semver, ext-state stamp, show-version action

Version "0.9.01" sourced once from CMake via configure_file'd header; pure
app_version module (parse/compare/classify) with CTest. Stamp rides the persist
save seam under "reasampler"/"version"; absent stamp reads as pre-versioning.
On-demand "show version" action, no startup print.

src/persist.cpp

@@ -79,6 +79,7 @@
 #include <string>
 #include <vector>
 
+#include "app_version.h"
 #include "capture_paths.h"
 
 #define REAPERAPI_MINIMAL
@@ -226,6 +227,13 @@ bool ReaSamplerSession::saveToActiveProject() {
     SetProjExtState(static_cast<ReaProject*>(proj), kProjExtNamespace,
                     kProjExtOwnedKey, ownedJson.c_str());
 
+    // Phase V (V1): stamp the WRITING version — the build producing this save — under the
+    // version key, on the SAME seam as the keys above so the stamp and MarkProjectDirty
+    // stay paired (no drifting ad-hoc SetProjExtState). appVersion() is the one CMake-
+    // sourced constant; every save records the exact current build into the .rpp.
+    SetProjExtState(static_cast<ReaProject*>(proj), kProjExtNamespace,
+                    kProjExtVersionKey, appVersion().c_str());
+
     MarkProjectDirty(static_cast<ReaProject*>(proj));
     return true;
 }
@@ -315,6 +323,16 @@ void ReaSamplerSession::loadFromProject(void* proj, const std::string& projectDi
     // (R-B) must re-read the restored manifest so it matches the rolled-back bank state.
     owned_ = loadOwnedManifest(static_cast<ReaProject*>(proj));
 
+    // Phase V (V1): recover the writing-version stamp on EVERY load path (peer-symmetry
+    // with tail_/view_ above). An absent stamp classifies as PreVersioning, a malformed
+    // one as Unknown — both silent, no console warning (a pre-versioning project is not
+    // an error). getProjExtStateString returns "" for an absent key, which is exactly the
+    // PreVersioning input classifyWritingVersion expects. proj == nullptr -> "" -> default.
+    writingVersion_ = classifyWritingVersion(
+        proj ? getProjExtStateString(static_cast<ReaProject*>(proj), kProjExtNamespace,
+                                     kProjExtVersionKey)
+             : std::string{});
+
     if (!proj) {
         book_ = BankBook{};
         return;