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/version_generated.h.in

@@ -0,0 +1,9 @@
+#pragma once
+// version_generated.h.in — configure_file TEMPLATE. CMake substitutes @REASAMPLER_VERSION@
+// (the ONE source-of-truth variable in CMakeLists.txt) and writes the result to the build
+// tree as version_generated.h. DO NOT edit the generated header — edit REASAMPLER_VERSION
+// in CMakeLists.txt; that single edit re-generates this and re-threads the exact string
+// (leading zero preserved verbatim) into the binary constant, the ext-state stamp, and the
+// show-version action. See app_version.h / .cpp.
+
+#define REASAMPLER_VERSION_STRING "@REASAMPLER_VERSION@"