bread-theme: shared component stylesheet + generator CLI

Adds the single source of truth for bread GUI styling so the apps stop
each re-implementing (and drifting on) component CSS:

- stylesheet(&Palette): full component sheet (buttons, entries, switches,
  dropdowns, lists/rows/sidebars, cards, chips, scrollbars, headings) built
  from the design tokens + a canonical @define-color block (surface=color0,
  overlay=color7, accent=color4).
- render() / shared_css_path() / write_shared_css(): render for the current
  pywal palette and write to $XDG_RUNTIME_DIR/bread/theme.css.
- gtk::apply_shared(): load that file (or a rendered fallback) at APPLICATION
  priority and watch it, so every app recolours live with no rebuild.
- new `bread-theme` CLI (generate|path|print) — gtk-free, light. Run at
  session start and on palette change; apps pick it up via the file watch.

The contract is a CSS *file*, so apps stay decoupled from this crate's gtk4
version. Tests cover the stylesheet, path, and render helpers.

bread-theme/src/bin/bread-theme.rs

@@ -0,0 +1,50 @@
+//! `bread-theme` — generates the ecosystem's shared GTK stylesheet from the
+//! current pywal palette and writes it to the canonical path that every bread
+//! GUI loads. Run it at session start, and again after the wallpaper/palette
+//! changes (e.g. from a pywal hook); apps watch the file and recolour live.
+//!
+//!   bread-theme            # same as `generate`
+//!   bread-theme generate   # render + write the shared stylesheet
+//!   bread-theme path       # print the stylesheet path
+//!   bread-theme print      # render to stdout (no write)
+
+use std::process::ExitCode;
+
+fn main() -> ExitCode {
+    let cmd = std::env::args().nth(1).unwrap_or_else(|| "generate".into());
+    match cmd.as_str() {
+        "path" => {
+            println!("{}", bread_theme::shared_css_path().display());
+            ExitCode::SUCCESS
+        }
+        "print" => {
+            print!("{}", bread_theme::render());
+            ExitCode::SUCCESS
+        }
+        "generate" => match bread_theme::write_shared_css() {
+            Ok(path) => {
+                eprintln!("bread-theme: wrote {}", path.display());
+                ExitCode::SUCCESS
+            }
+            Err(e) => {
+                eprintln!("bread-theme: failed to write stylesheet: {e}");
+                ExitCode::FAILURE
+            }
+        },
+        "-h" | "--help" | "help" => {
+            eprintln!(
+                "bread-theme — shared stylesheet generator\n\n\
+                 USAGE:\n  bread-theme [generate|path|print]\n\n\
+                 generate  render the pywal palette to the shared stylesheet (default)\n\
+                 path      print the stylesheet path ({})\n\
+                 print     render to stdout without writing",
+                bread_theme::shared_css_path().display()
+            );
+            ExitCode::SUCCESS
+        }
+        other => {
+            eprintln!("bread-theme: unknown command '{other}' (try generate|path|print)");
+            ExitCode::FAILURE
+        }
+    }
+}