row 198 r3: BUILD.md records N2 as a named follow-up (sage-2868)

boardSend's /api/seen catch returns 400 for a scan failure after writeMark.
Ruled left as built; it goes to Sage's post-253 DEFERRED sweep. Doc-only.

agents/bushroot/work/queue-198/BUILD.md

@@ -168,6 +168,8 @@ match a real spec.
   the Console; exit 0 is scan only.
 - `docs/plans/CURRENT.md`: one dated line appended at the end (append-only).
 
+Open, ruled sage-2868: boardSend's `/api/seen` catch returns 400 for a scan failure after writeMark. Left as built: a scan failure is not a client error, so this is a real gap. Named follow-up in Sage's post-253 DEFERRED sweep (N2).
+
 Still in the README, on purpose: `packages/webui/README.md` line 826 runs the control-board suite
 (`node --test packages/control-board/tests/`). The suite still exists, so the line stays.