Give Effector a version and a changelog, shown from the menu

The version is the project() line of CMakeLists.txt and nowhere else:
CMake writes it, with the text of CHANGELOG.md, into a generated
core/version.hpp, so the number and the release notes the binary shows
are the ones it was built from. effector --version (-V) prints it, and
the header-bar menu gains a section with Changelog, a window laying the
Markdown out as headings and bullets, and About Effector.

The core tests hold the newest changelog entry to the current version,
so a bump cannot go in without its notes. Releases are tagged vX.Y.Z;
this is 0.5.0, and the changelog reconstructs 0.1.0-0.4.0 from the
history.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CLAUDE.md

@@ -33,6 +33,20 @@ Headless render (no display needed, the fastest end-to-end smoke test):
 build/effector -o /tmp/out.png -s 800x600 --strip-height 6 --seed 1 a.jpg b.png
 ```
 
+### Versions and the changelog
+
+The version is the `project()` line of `CMakeLists.txt`, and nowhere else.
+CMake writes it, together with the text of `CHANGELOG.md`, into a generated
+`core/version.hpp` (`kVersion`, `kChangelog`, from `src/core/version.hpp.in`)
+in the build tree. `--version` prints it, and the header-bar menu shows it
+under About Effector and the changelog under Changelog. The `version` core test
+fails unless the newest `## [X.Y.Z]` entry in `CHANGELOG.md` is the current
+version, so a bump and its changelog entry go in together. Each release is an
+annotated `vX.Y.Z` tag on the commit that made it. When you add a user-visible
+change, add a line to the newest changelog entry, or start a new entry if that
+one is already tagged. The Changelog window reads only headings, `- ` bullets
+(wrapped onto indented lines) and `` `code` ``, so keep the file to those.
+
 ### Running the GUI from this shell
 
 GTK picks Wayland from an inherited `WAYLAND_DISPLAY` and will not draw into an