Set the version once, in one place, for every artefact

The workspace read 0.1.0 for two releases, so every desktop binary reported a
version two releases stale. The APK was worse: `AndroidManifest.xml` states no
version at all, so a device showed `versionName=null` and `versionCode=0` while
the library inside the APK knew exactly what it was. A version edited by hand
in several files is a version that is wrong in at least one of them.

`tools/set-version.sh` is now the only thing that sets one. It takes the
version from the latest git tag, or is told, and writes the two files that must
state it before anything is built: the workspace `Cargo.toml`, from which every
crate inherits, and `packaging/PKGBUILD`, which pacman reads before a build
exists. It refreshes `Cargo.lock`, because members appear there by version and
CI builds `--locked`. `--commit` commits the result.

Android is not in that list on purpose. `package.sh` reads the version out of
`Cargo.toml` and hands it to `aapt2 link`, so the APK cannot drift from the
binary it contains — there is no third file to forget. `versionCode` has to be
one increasing integer, which a semantic version is not, so it is packed as
MAJOR*10000 + MINOR*100 + PATCH: ordered the way Android requires, and readable
at a glance.

A version that is not MAJOR.MINOR.PATCH is refused rather than coerced. It is
a contract with whoever reads a bug report, and silently turning "0.4" into
something else is worse than being asked to type it again.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

packaging/PKGBUILD

@@ -4,7 +4,7 @@
 # makes `makepkg -si` in this directory install what you are actually working
 # on. Swap `source` for a tagged tarball when there is something to release.
 pkgname=darkroom
-pkgver=0.3.0
+pkgver=0.4.0
 pkgrel=1
 pkgdesc="Non-destructive RAW photo library and editor"
 arch=('x86_64')