docs(sales-summaries): prove the re-key holds with both clients active

Re-activated all 10 twins so every Square location is shared again, then
re-imported. Three consecutive refund imports for both halves of the Concord
pair leave counts unchanged: NGCC 127/127/127/127, NGCD 1419/1419/1419/1419.

That is the configuration that previously made a refund's owner flip every ~20
minutes. With client-scoped keys each client resolves its own entity and updates
it in place, so repeated imports are idempotent and the two copies are stable.

Only the first import after migration grows anything, once: NGCD materialised
100 of its own copies while NGCC, already carrying its scoped key, gained only
the 2 refunds that were genuinely new.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/2026-08-15-sales-summary-balance-validation.md

@@ -287,3 +287,27 @@ came to $56.37 across 69 days — identical to the in-memory prediction in Part
 the job path end to end. The 20 contended clients were re-measured in memory after the charge
 repair, because a second full pass through the job would have taken hours for the reason in defect
 2. The two methods agree exactly where both were run.
+
+## Ongoing deduplication, with both clients active again
+
+The re-key is meant to make contention impossible even if a location is configured on two clients
+again. To test that rather than assume it, all 10 twins were **re-activated** — every location
+shared once more — and the import re-run.
+
+**Repeated imports are idempotent.** Three consecutive refund imports for both halves of the
+Concord pair, with both clients live on location `L43Z5GMW72VMG`:
+
+| | run 0 | run 1 | run 2 | run 3 |
+|---|---|---|---|---|
+| NGCC | 127 | 127 | 127 | 127 |
+| NGCD | 1,419 | 1,419 | 1,419 | 1,419 |
+
+Not one entity created or moved. Before the re-key this is the exact configuration that made a
+refund's owner flip every ~20 minutes.
+
+The first import after the migration is the only one that grows anything, and it grows once: NGCD
+gained 100 refunds materialising its own copies, while NGCC — whose refunds already carried its
+own scoped key — gained only the 2 that were genuinely new. After that, steady state.
+
+**State the restore was left in:** twins re-activated, i.e. Phase 0 deliberately undone for this
+test. Re-applying it is the ten-datom retraction recorded above.