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.h

@@ -19,6 +19,7 @@
 
 #include <string>
 
+#include "app_version.h"
 #include "bank_book.h"
 #include "bank_model.h"
 #include "owned_manifest.h"
@@ -70,6 +71,14 @@ inline constexpr const char* kProjExtTailKey = "tail_setting";
 // graceful, but the attribution safety net is lost until the next capture rebuilds it).
 inline constexpr const char* kProjExtOwnedKey = "owned_files";
 
+// The ext-state key holding the ReaSampler version that last WROTE this project
+// (Phase V, V1). Written on every save alongside the banks/view/tail keys, so every
+// saved .rpp records which build produced its state — the seam a future within-channel
+// forward migration keys off ("this was written by 0.9.01, I am 0.9.05"). An absent
+// key is the explicit pre-versioning case (a project saved before this shipped), read
+// silently, never an error. FOREVER-STABLE key string once shipped.
+inline constexpr const char* kProjExtVersionKey = "version";
+
 // 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
@@ -143,6 +152,15 @@ public:
     OwnedFileManifest& owned() { return owned_; }
     const OwnedFileManifest& owned() const { return owned_; }
 
+    // The ReaSampler version that last WROTE the active project, recovered from its
+    // ext-state stamp on load (Phase V, V1). PreVersioning when the project carries no
+    // stamp (saved before this feature), Unknown for a malformed stamp, Stamped with the
+    // exact stored string otherwise — all silent, never an error. Replaced on every load
+    // path (peer-symmetry with bank_/view_/tail_); default PreVersioning for an unsaved
+    // or never-loaded session. Exposed so a future migration step (or diagnostics) can
+    // reason about the origin build without re-reading ext state.
+    const WritingVersion& writingVersion() const { return writingVersion_; }
+
     // Serialize the current book (under the `banks` key), view model, and tail setting
     // to the active project's ext state (namespace "reasampler"), and clear the retired
     // legacy `bank_index` key. Non-destructive beyond writing our own ext-state keys.
@@ -208,6 +226,12 @@ private:
     // the in-memory set matches disk. Absent key -> empty is graceful (older project).
     OwnedFileManifest owned_;
 
+    // The writing-version stamp recovered on load (Phase V). Default PreVersioning;
+    // loadFromProject replaces it on every load path (peer to bank_/view_/tail_), so
+    // switching to a pre-versioning project reports PreVersioning rather than inheriting
+    // the previous project's stamp. Read-only to consumers via writingVersion().
+    WritingVersion writingVersion_;
+
     // 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