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>

README.md

@@ -25,6 +25,7 @@ installs it to `~/.local/bin`.
 build/effector                  # GTK interface
 build/effector --list-effects
 build/effector --help
+build/effector --version
 ```
 
 Headless rendering, no display needed:
@@ -60,6 +61,17 @@ batch. Small batches finish before the window would be drawn, so it only appears
 if the decode is still going after 250 ms. On a terminal the headless mode keeps
 a single `reading n/total` line updated on stderr; redirected output stays clean.
 
+## Versions
+
+Releases are listed in [CHANGELOG.md](CHANGELOG.md) and tagged `vX.Y.Z`
+(`git tag -l 'v*'`). `effector --version` prints the version, and the menu in
+the interface has Changelog and About Effector.
+
+To cut a release: bump the `project()` version in `CMakeLists.txt`, move the
+changes into a new `## [X.Y.Z] - date` entry at the top of `CHANGELOG.md` (the
+core tests fail until the two agree), commit, and tag it:
+`git tag -a vX.Y.Z -m "Effector X.Y.Z"`.
+
 ## Effects
 
 ### scanlines