workspace: add bread-app crate and first-cut bread-polkit agent

bread-app is the GTK bootstrap new tools should use instead of another
copied main.rs: com.breadway.* app id, singleton lock, optional
gtk_popup re-export, optional bread.command.<app>.** listen loop.
Tests cover app-id helpers and command-verb parse. Existing apps are
not migrated.

bread-polkit is an own PolicyKit1 session authentication agent with a
bread-theme GTK4 password prompt (not a polkit-gnome wrapper).
Autostart via contrib/bread-polkit.desktop or exec-once. Not a bakery
product; not added to the BOS ISO lockfile.

bread-app/src/command.rs

@@ -0,0 +1,121 @@
+//! Command-bus helpers for `bread.command.<app>.**`.
+//!
+//! The `command_id` here is the breadd sibling-app id (`clip`, `box`,
+//! `shot`) — often shorter than the GTK / singleton name (`breadclip`).
+
+use crate::id::{parse_app_name, InvalidAppId};
+use bread_utils::bread_client::{BreadClient, BreadEvent, Subscription};
+
+/// Same charset as [`parse_app_name`]: a single command-bus segment.
+pub fn parse_command_id(command_id: &str) -> Result<&str, InvalidAppId> {
+    parse_app_name(command_id)
+}
+
+/// Subscribe glob: `bread.command.<app>.**`.
+pub fn command_pattern(command_id: &str) -> Result<String, InvalidAppId> {
+    let id = parse_command_id(command_id)?;
+    Ok(format!("bread.command.{id}.**"))
+}
+
+/// The verb segment of `bread.command.<app>.<verb>` (and extra trailing
+/// segments, if any). `None` when the event is not addressed to
+/// `command_id` or the verb is missing.
+///
+/// Extra dotted remainder (`bread.command.clip.stack.clear`) yields the
+/// first remaining segment (`stack`) — a verb is one segment, matching
+/// [`BreadClient::command`].
+pub fn command_verb<'a>(event: &'a str, command_id: &str) -> Option<&'a str> {
+    if command_id.is_empty() {
+        return None;
+    }
+    let prefix = format!("bread.command.{command_id}.");
+    let rest = event.strip_prefix(&prefix)?;
+    let verb = rest.split('.').next()?;
+    if verb.is_empty() {
+        None
+    } else {
+        Some(verb)
+    }
+}
+
+/// Subscribe to `bread.command.<command_id>.**` and invoke `on_verb` with
+/// the parsed verb plus the raw event.
+///
+/// Fail-silent: constructing the client and holding the subscription never
+/// requires breadd to be running. Drop the returned [`Subscription`] (or
+/// call [`Subscription::stop`]) to end the loop.
+pub fn listen_commands<F>(command_id: &str, on_verb: F) -> Result<Subscription, InvalidAppId>
+where
+    F: Fn(&str, BreadEvent) + Send + 'static,
+{
+    let id = parse_command_id(command_id)?.to_string();
+    let client = BreadClient::connect(id.clone());
+    let pattern = format!("bread.command.{id}.**");
+    Ok(client.subscribe(pattern, move |event| {
+        let Some(verb) = command_verb(&event.event, &id).map(str::to_owned) else {
+            return;
+        };
+        on_verb(&verb, event);
+    }))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn command_pattern_uses_double_star() {
+        assert_eq!(command_pattern("clip").unwrap(), "bread.command.clip.**");
+        assert_eq!(command_pattern("shot").unwrap(), "bread.command.shot.**");
+    }
+
+    #[test]
+    fn command_pattern_rejects_invalid_id() {
+        assert!(command_pattern("").is_err());
+        assert!(command_pattern("clip.clear").is_err());
+    }
+
+    #[test]
+    fn command_verb_strips_app_prefix() {
+        assert_eq!(
+            command_verb("bread.command.clip.clear", "clip"),
+            Some("clear")
+        );
+        assert_eq!(
+            command_verb("bread.command.shot.region", "shot"),
+            Some("region")
+        );
+        assert_eq!(
+            command_verb("bread.command.shot.annotate", "shot"),
+            Some("annotate")
+        );
+    }
+
+    #[test]
+    fn command_verb_takes_first_segment_only() {
+        assert_eq!(
+            command_verb("bread.command.clip.stack.clear", "clip"),
+            Some("stack")
+        );
+    }
+
+    #[test]
+    fn command_verb_rejects_other_apps_and_missing_verb() {
+        assert_eq!(command_verb("bread.command.clip.clear", "shot"), None);
+        assert_eq!(command_verb("bread.command.clip", "clip"), None);
+        assert_eq!(command_verb("bread.command.clip.", "clip"), None);
+        assert_eq!(command_verb("bread.clip.copied", "clip"), None);
+        assert_eq!(command_verb("bread.command.clip.clear", ""), None);
+    }
+
+    #[test]
+    fn listen_commands_rejects_invalid_id() {
+        assert!(listen_commands("", |_, _| {}).is_err());
+    }
+
+    #[test]
+    fn listen_commands_stop_joins_without_a_daemon() {
+        let sub = listen_commands("clip", |_, _| {}).unwrap();
+        sub.stop();
+    }
+}