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.

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"
 }