Add channels metadata: membership and build-channel rules for edge/rc/stable

A package's .omarchy/package.json may now pin where it lives with
"channels": [...]. With the key present the package builds in each listed
channel except stable (stable is only fed by promotion); without it, today's
defaults hold (build for edge; fast-ring also builds stable directly).

package_moves_to_channel() is the advance/promote eligibility rule: a member
of the destination channel that is not built there natively.

The release pair (omarchy, omarchy-settings) is edge+rc+stable — edge stays
during the client-migration overlap window and drops later. The dev pair is
pinned to edge only.

helpers/package-metadata.sh

@@ -12,6 +12,8 @@
 #   { "source": "aur", "pkgrel": { "suffix": 1, "offset": 1 } }
 #   { "source": "aur", "rebuild_on": ["qt6-base"] }
 #   { "source": "local" }
+#   { "source": "local", "channels": ["edge"] }
+#   { "source": "local", "channels": ["edge", "rc", "stable"] }
 #   { "source": "local", "min_release_age": "24h" }
 #   { "source": "local", "upstream": { "github": "owner/repo", "checksums": "SHASUMS256.txt", "assets": { "x86_64": "name-{tag}-x64.tar.xz" } } }
 #
@@ -138,6 +140,28 @@ package_has_pkgbuild() {
   [[ -f "$pkgdir/PKGBUILD" ]]
 }
 
+# Channel membership: where a package may be published. Packages without a
+# `channels` key are members of every channel (they flow edge -> rc -> stable).
+package_has_channels() {
+  local pkgdir="$1" metadata
+  metadata=$(metadata_file_for_dir "$pkgdir")
+  [[ -f "$metadata" ]] || return 1
+  jq -e 'has("channels")' "$metadata" >/dev/null
+}
+
+package_channels() {
+  local pkgdir="$1" metadata
+  metadata=$(metadata_file_for_dir "$pkgdir")
+  [[ -f "$metadata" ]] || return 0
+  jq -r '(.channels // [])[]' "$metadata"
+}
+
+package_in_channel() {
+  local pkgdir="$1" channel="$2"
+  package_has_channels "$pkgdir" || return 0
+  package_channels "$pkgdir" | grep -qx "$channel"
+}
+
 package_builds_for_mirror() {
   local pkgdir="$1"
   local mirror="$2"
@@ -145,6 +169,23 @@ package_builds_for_mirror() {
   package_has_pkgbuild "$pkgdir" || return 1
   package_has_metadata "$pkgdir" || return 1
 
+  # An explicit `channels` key pins where a package is BUILT: it builds in
+  # each listed channel except stable, which is only ever fed by promotion.
+  # Without the key, the defaults hold: everything builds for edge, and
+  # fast-ring packages also build directly for stable (bin/release then
+  # replicates those artifacts into rc to keep rc and stable in parity).
+  if package_has_channels "$pkgdir"; then
+    case "$mirror" in
+      edge | rc)
+        package_in_channel "$pkgdir" "$mirror"
+        return
+        ;;
+      *)
+        return 1
+        ;;
+    esac
+  fi
+
   case "$mirror" in
     edge)
       return 0
@@ -158,6 +199,16 @@ package_builds_for_mirror() {
   esac
 }
 
+# Whether `bin/repo advance` may carry this package into the given channel:
+# a member of that channel that is not built there natively. Native builds are
+# authoritative — advancing over them could pair a published filename with
+# different bytes, which the R2 cache would never recover from.
+package_moves_to_channel() {
+  local pkgdir="$1" channel="$2"
+  package_in_channel "$pkgdir" "$channel" || return 1
+  ! package_builds_for_mirror "$pkgdir" "$channel"
+}
+
 package_dirs() {
   [[ -d "$PKGBUILDS_DIR" ]] || return 0
 
@@ -361,6 +412,17 @@ validate_package_metadata() {
     *) echo "invalid release_ring for $(basename "$pkgdir"): $ring"; return 1 ;;
   esac
 
+  if ! jq -e '
+    if has("channels") | not then true
+    else .channels | type == "array" and length > 0
+      and all(. == "edge" or . == "rc" or . == "stable")
+      and (unique | length) == length
+    end
+  ' "$metadata" >/dev/null; then
+    echo "invalid channels for $(basename "$pkgdir"): must be a non-empty array of unique edge/rc/stable values"
+    return 1
+  fi
+
   if ! package_min_release_age_seconds "$pkgdir" >/dev/null; then
     echo "invalid min_release_age for $(basename "$pkgdir"): must be a number with optional s/m/h/d suffix"
     return 1