sync push --branch: two dev-only reference edge cases after a promote #928

Problem

After the promote fix (sync push --branch DEV from main/), two edge cases can still leave a reference on the dev branch pointing at a config or row that no longer exists there. Both affect only the dev branch; production data is not touched.

GPT Astra found both during the final review of the promote fix, and both were reproduced with in-memory calls.

1. A promoted row deleted on both sides still resolves through its saved mapping.
   • Steps: promote P/r1, which creates D/d1 on DEV. Remove the local r1 row folder and delete d1 on DEV. Edit a flow to select P/["r1"] and push --branch DEV.
   • Neither side contains the row, so the row changeset is empty and note_gone() never sees the deletion.
   • Result: the DEV flow gets D/["d1"], and no pending marker is kept.
   • Location: services/_sync_promote_remap.py (the _unit / note_gone path).
   • Fix idea: validate saved mappings against the target inventory, including tracked targets that are absent from both sides of the diff.
2. A deleted identity-mapped parent loses its pending marker when the task selects no rows.
   • Steps: the dev branch held config P under the production id and P was later deleted on DEV. A pending flow task runs P without configRowIds.
   • The rewrite is correctly blocked, but open_refs() returns nothing, so recovery clears rebind_pending.
   • Location: services/_sync_promote_remap.py (open_refs).
   • Fix idea: pending detection should account for known-deleted parents, also for identity mappings without row selections.