advance: skip same-name files with differing bytes instead of failing

Incremental advances collide with same-version artifacts that reached the
destination through another lineage — the stable -> rc bootstrap seed and
native rc builds are not byte-identical to edge's builds of the same
version. A collision means the package has not moved in the source since,
so keep the destination's published copy and continue; the hard failure
also aborted before the db rebuild and sync, leaving the advance half-done.

Also stop advancing pinned packages into rc: eligibility deferred to
package_builds_for_mirror, whose OMARCHY_RC_PINS gate is never set during
an advance, so edge's omarchy/omarchy-settings looked movable — and could
have overtaken an in-flight RC pin under a fresh filename.

bin/advance-channel

@@ -326,7 +326,7 @@ while IFS=$'\t' read -r name base filename; do
 done < <(read_manifest)
 
 echo ""
-print_info "Copied: $COPIED | Already present: $PRESENT | Not eligible: $SKIPPED"
+print_info "Copied: $COPIED | Already present: $PRESENT | Differing (kept): ${#DIFFERING[@]} | Not eligible: $SKIPPED"
 
 if [[ ${#MISSING_FILES[@]} -gt 0 ]]; then
   print_error "${#MISSING_FILES[@]} package(s) named in the $FROM database are missing on disk:"
@@ -345,13 +345,12 @@ if [[ ${#MISSING_SIGS[@]} -gt 0 ]]; then
 fi
 
 if [[ ${#DIFFERING[@]} -gt 0 ]]; then
-  print_error "${#DIFFERING[@]} file(s) already published in $TO with DIFFERENT bytes:"
+  print_warning "${#DIFFERING[@]} file(s) already published in $TO with different bytes — kept as-is:"
   printf '  %s\n' "${DIFFERING[@]}"
-  echo "Published filenames are never rewritten, and two artifacts fighting over"
-  echo "one name means something built twice from different inputs. Resolve it"
-  echo "deliberately: bump pkgrel and rebuild so the new artifact gets a new"
-  echo "filename, or remove the source copy if the destination is correct."
-  exit 1
+  echo "Published filenames are never rewritten, so the $TO copy stays authoritative."
+  echo "A same-name collision means this version reached $TO through another lineage"
+  echo "(native build, bootstrap seed, or a same-version rebuild) and the package has"
+  echo "not moved in $FROM since; it advances under a new filename when it does."
 fi
 
 if [[ "$DRY_RUN" == true ]]; then

helpers/package-metadata.sh

@@ -269,6 +269,11 @@ package_builds_for_mirror() {
 package_moves_to_channel() {
   local pkgdir="$1" channel="$2"
   package_in_channel "$pkgdir" "$channel" || return 1
+  # Pinned packages build natively in rc from the release pin. That the
+  # advancing environment lacks OMARCHY_RC_PINS (so *it* may not build them)
+  # does not make the edge copy movable over the pin's artifact — edge's
+  # version can be ahead of the in-flight RC.
+  [[ "$channel" == "rc" ]] && package_is_pinned "$pkgdir" && return 1
   ! package_builds_for_mirror "$pkgdir" "$channel"
 }