bakery: add stable/beta/dev build tracks

Adds a track concept to bakery (separate from the existing bakery/pacman
distribution channel): stable (unchanged tag-triggered releases), beta
(deliberate beta-v* tag promotion), and dev (published on every push to
dev). Each track gets its own signed index + artifact tree under
dl.breadway.dev so stable's paths and existing installs are untouched.

- bakery: new Track type, a global track preference in installed.json
  (defaults to stable via serde, no migration needed), `bakery track
  show`/`set`, a BAKERY_INDEX_BASE_URL override for testing, and a real
  semver comparison in `update` (was a plain string-equality check before).
  ANSI-colored/aligned CLI output (TTY + NO_COLOR aware).
- gen-index.sh: TRACK env var selects which subtree to read/write.
- CI: dev-bakery.yml/beta-bakery.yml/dev-bread-theme.yml/beta-bread-theme.yml
  publish those two products on the new tracks; dev/beta skip the GitHub
  Release upload step (no per-commit release spam).
- docs/release-channels.md documents the three-track policy.

bakery/src/state.rs

@@ -1,3 +1,4 @@
+use crate::track::Track;
 use anyhow::{Context, Result};
 use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
@@ -14,6 +15,11 @@ pub struct InstalledPackage {
 
 #[derive(Debug, Default, Deserialize, Serialize)]
 pub struct State {
+    // `#[serde(default)]` lets an installed.json written by a pre-track
+    // bakery binary deserialize straight into Track::Stable with no
+    // migration step.
+    #[serde(default)]
+    pub track: Track,
     pub packages: HashMap<String, InstalledPackage>,
 }
 
@@ -52,6 +58,10 @@ impl State {
     pub fn remove(&mut self, name: &str) -> Option<InstalledPackage> {
         self.packages.remove(name)
     }
+
+    pub fn set_track(&mut self, track: Track) {
+        self.track = track;
+    }
 }
 
 fn state_path() -> PathBuf {
@@ -101,6 +111,23 @@ mod tests {
         assert!(state.remove("nope").is_none());
     }
 
+    #[test]
+    fn track_defaults_to_stable_on_old_shape_json() {
+        // Simulates installed.json written before the track field existed.
+        let old_shape = r#"{"packages":{}}"#;
+        let state: State = serde_json::from_str(old_shape).unwrap();
+        assert_eq!(state.track, Track::Stable);
+    }
+
+    #[test]
+    fn set_track_updates_and_roundtrips() {
+        let mut state = State::default();
+        state.set_track(Track::Dev);
+        let json = serde_json::to_string(&state).unwrap();
+        let restored: State = serde_json::from_str(&json).unwrap();
+        assert_eq!(restored.track, Track::Dev);
+    }
+
     #[test]
     fn json_roundtrip() {
         let mut state = State::default();