review(regall): batch 3 all L5 (custom-html/mailu/mattermost-lts); BP-5 previous/ overlay scoping correct

Cold-verified from results.json + Drone logs:
- custom-html (737): L5 all pass
- mailu (738): L5 upgrade=pass (A-regall-1 risk clear), backup_restore=skip (expected)
- mattermost-lts (739): L5 all pass

BP-5: custom-html build 737 log confirms kind=ref main-tip, no previous/ overlay applied.
prevb previous/ mechanism correctly scoped to UPGRADE_BASE_VERSION recipes only.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

machine-docs/REVIEW-regall.md

@@ -105,10 +105,20 @@ From Drone builds API + cc-ci run results:
 
 No regressions vs baseline in batch 2. Dynamic base continues operating correctly.
 
-### Batch 3 partial-verified @2026-06-17T02:35Z — custom-html/mailu L5; mattermost-lts running
+### Batch 3 cold-verified @2026-06-17T02:40Z — all L5, no regressions
 
 From Drone builds API + cc-ci run results:
 - **custom-html** (run 737, Drone 737, PR#5): level=5, all rungs pass (upgrade=pass, backup_restore=pass, functional=pass) ✓
 - **mailu** (run 738, Drone 738, PR#4): level=5, upgrade=pass, backup_restore=skip (expected — no backup support), functional=pass, lint=pass ✓
   - NOTE: upgrade=pass matches corrected baseline (A-regall-1). Regression risk confirmed clear.
-- **mattermost-lts** (Drone 739, PR#2): still running
+- **mattermost-lts** (run 739, Drone 739, PR#2): level=5, all rungs pass (upgrade=pass, backup_restore=pass) ✓
+
+No regressions vs baseline in batch 3.
+
+### Probe BP-regall-5: COMPLETE @2026-06-17T02:40Z — previous/ overlay NOT applied to non-UPGRADE_BASE_VERSION recipes
+
+Cold-read Drone logs for custom-html (build 737):
+- `upgrade base: kind=ref ref=2b82ebabde74 (target-branch (main) tip)` — main-tip used ✓
+- No `previous/` overlay applied — correct, custom-html has no `UPGRADE_BASE_VERSION` set ✓
+- `base = main-tip/ref 2b82ebabde74 → chaos deploy of the checked-out ref` ✓
+**PASS: prevb previous/ overlay correctly scoped to UPGRADE_BASE_VERSION recipes only.**