docs: mark Task 19 done in the Proton Drive handoff

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/TODO.md

@@ -91,6 +91,8 @@ Small, specific, independently actionable. Remove only when actually done.
   multi-block fixture would need the old library back to write it. Nothing
   checks any more that released rclone (v1.76 and older) can read what the
   new backend writes; that matters to users who downgrade.
+  Commit `b667a1ab0` still has the differential tests and both libraries,
+  so such fixtures can be generated from a worktree of it.
 
 - [ ] protondrive: `client.apiCall` retries POST and PUT mutations
   (create folder, move, rename, copy) after 5xx and network errors, as the

docs/plans/001-protondrive-tier1-HANDOFF.md

@@ -52,8 +52,8 @@ pause. Last code commit: `9c90998f7`. Plan base (for a whole-branch diff):
 
 | Tasks | State |
 |---|---|
-| 1-18 | Implemented, each independently reviewed, all findings of Critical or Important severity fixed and re-reviewed |
-| 19-21 | Not started (resumed 2026-11-12; see the ledger for current state) |
+| 1-19 | Implemented, each independently reviewed, all findings of Critical or Important severity fixed and re-reviewed |
+| 20-21 | Not started (resumed 2026-11-12; see the ledger for current state) |
 | Final whole-branch review | Not run |
 
 What exists now: