bread-theme: generate Hyprland layer rules from the shell theme (Phase 4a)

Adds `bread-theme layerrules`, which writes the active shell theme's
[compositor] table to ~/.config/hypr/layerrules.json (atomic write). This
lets ~/.config/hypr/scripts/ui/rules.lua read theme-driven blur/transparency/
animation for the breadbar/breadbox layer-shell namespaces instead of having
them hardcoded, following THEME_SYSTEM_PLAN.md §9. rules.lua keeps its
previous hardcoded rules as a pcall-guarded fallback for when the JSON is
missing or malformed (lives outside this repo, so not part of this commit).

Also fixes a pre-existing test race: shell::tests and the new
layerrules::tests both mutate XDG_CONFIG_HOME in parallel `cargo test`
threads but previously used separate, unrelated locks (or none), so they
could observe each other's env var changes mid-test. Both now share
bread_theme::test_support::XDG_CONFIG_HOME_LOCK.

bread-theme/src/output.rs

@@ -54,7 +54,7 @@ pub fn output_palette_path(output: &str) -> PathBuf {
     palettes_dir().join(format!("{}.json", sanitize_output(output)))
 }
 
-fn atomic_write(path: &Path, contents: &str) -> std::io::Result<()> {
+pub(crate) fn atomic_write(path: &Path, contents: &str) -> std::io::Result<()> {
     if let Some(parent) = path.parent() {
         std::fs::create_dir_all(parent)?;
     }