Point package managers at China mirrors on cn installs

npm, PyPI and uv, Go, rustup and crates.io, RubyGems and Bundler,
and Maven reach their registries slowly or not at all from China.
On a cn install, link the profile's environment, cargo and gem
drop-ins into place. A regular file the admin wrote is never
replaced; the install fails instead.

docs/regions.md

@@ -34,7 +34,11 @@ For China, the builder installs `archlinuxcn-keyring` using the existing Arch tr
 
 ## Scope of the China profile
 
-The China profile provides repository defaults only. Language is a separate installer choice, not part of a region: a Chinese speaker outside China keeps global mirrors, and a China install in English still gets the regional ones.
+The China profile provides repository defaults and developer mirrors. Language is a separate installer choice, not part of a region: a Chinese speaker outside China keeps global mirrors, and a China install in English still gets the regional ones.
+
+## Developer mirrors
+
+The `cn` profile also points the common package managers at mirrors inside China: `environment.conf` (npm, PyPI and uv, Go, rustup, Bundler, and Maven through `MAVEN_ARGS`), `cargo.toml` for crates.io, `gemrc` for RubyGems, and `maven-settings.xml`, which Maven reads in place. During installation `install/config/region.sh` links the first three to `/etc/environment.d/90-omarchy-china.conf`, `/.cargo/config.toml` and `/etc/gemrc` when the marker says `cn`. It refuses to replace a regular file it did not create. The environment reaches sessions started after login.
 
 ## Remaining release validation