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/track.rs

@@ -0,0 +1,90 @@
+use clap::ValueEnum;
+use serde::{Deserialize, Serialize};
+use std::fmt;
+use std::str::FromStr;
+
+/// Which build of a package bakery follows: the tagged stable release, a
+/// deliberately-promoted beta, or the continuously-published `dev` branch
+/// build. Not to be confused with the *distribution* channel (bakery vs.
+/// pacman) documented in `docs/release-channels.md` — that's an orthogonal,
+/// pre-existing use of the word "channel", which is why this is called a
+/// "track" instead.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Deserialize, Serialize, ValueEnum)]
+#[serde(rename_all = "lowercase")]
+pub enum Track {
+    Stable,
+    Beta,
+    Dev,
+}
+
+impl Default for Track {
+    fn default() -> Self {
+        Track::Stable
+    }
+}
+
+impl Track {
+    pub fn as_str(&self) -> &'static str {
+        match self {
+            Track::Stable => "stable",
+            Track::Beta => "beta",
+            Track::Dev => "dev",
+        }
+    }
+}
+
+impl fmt::Display for Track {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(self.as_str())
+    }
+}
+
+impl FromStr for Track {
+    type Err = String;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s.to_lowercase().as_str() {
+            "stable" => Ok(Track::Stable),
+            "beta" => Ok(Track::Beta),
+            "dev" => Ok(Track::Dev),
+            other => Err(format!("unknown track '{other}' — expected stable, beta, or dev")),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn default_is_stable() {
+        assert_eq!(Track::default(), Track::Stable);
+    }
+
+    #[test]
+    fn display_roundtrips_through_from_str() {
+        for track in [Track::Stable, Track::Beta, Track::Dev] {
+            let s = track.to_string();
+            assert_eq!(s.parse::<Track>().unwrap(), track);
+        }
+    }
+
+    #[test]
+    fn from_str_is_case_insensitive() {
+        assert_eq!("DEV".parse::<Track>().unwrap(), Track::Dev);
+        assert_eq!("Beta".parse::<Track>().unwrap(), Track::Beta);
+    }
+
+    #[test]
+    fn from_str_rejects_unknown() {
+        assert!("nightly".parse::<Track>().is_err());
+    }
+
+    #[test]
+    fn json_roundtrip_uses_lowercase() {
+        let json = serde_json::to_string(&Track::Dev).unwrap();
+        assert_eq!(json, "\"dev\"");
+        let back: Track = serde_json::from_str(&json).unwrap();
+        assert_eq!(back, Track::Dev);
+    }
+}