row 198 r3: BUILD.md places the reply join killer (B3, M16)

The M16 killer is in serve.test.mjs, not board.test.mjs, because reply() refuses
an unregistered row before any send. Doc-only; no code or test change.

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

@@ -78,6 +78,9 @@ plan leaves state unchanged. The README rollback note covers any move to a pre-1
 - `packages/control-board/tests/board.test.mjs` (B3): new test, M15. After a write, `index()` must
   start a fresh scan. The event log must read `write, bump, scan, scan`, and the answer must show
   the mark.
+- The reply join killer (B3, M16) lives in `packages/control-board/tests/serve.test.mjs`, not
+  `board.test.mjs`. `reply()` refuses an unregistered row before any send, so the killer needs a
+  registered fixture (`replyFixture`), which `serve.test.mjs` already builds.
 - `packages/webui/tests/board-inprocess.test.mjs` (new, B3): M6 and M7 (preview: each board route
   answers 401 without the proxy header and 403 with a foreign Host, and the board never runs);
   M5 (`/api/seen` without `--send` writes no `seen.json`); M10 (positive control: with `--send` on