docs/plans/CHECKLIST.md: the checks before a plan is handed over; T41 to T43

One list for the design model to work through per task, gathering the
task-writing tips that apply before hand-over (T18, T19, T24 to T29,
T32 to T35, T39, T40) and three new ones from the combined Ornith
guidance: walk each given test by hand against the task's rules (T41),
grep earlier given files for anything the task changes (T42), and run
the gate's own checks on the given files (T43).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/implementer-lessons.md

@@ -78,6 +78,9 @@ How it is used:
 | T38 | A tool's answer that the model relays must say the outcome in plain words, including what did not happen ("not added to memory.md: …"). A label in parentheses reads as success. | M5a device check: the model twice told the owner "Added to memory.md" for a write that became an untrusted note. After task 22 it reported it correctly. |
 | T39 | Name the source directory of copied files in every task's Files section. | M5a task 06 logged a deviation about a files directory the task never named. |
 | T40 | Give the gate's shell timeout in the task (600000 ms). On a machine that also serves the model, the gate takes longer than the implementer's default of 2 minutes, and a timed-out gate looks like a failure. | M5a tasks 18 and 19 each ran the gate twice on titan; tasks 21 and 22, which gave the timeout, ran it once. |
+| T41 | Before hand-over, walk each given test by hand against the task's own rules. A replay of the reference proves the tests can pass; it cannot show that a test contradicts the task, because the reference was written to pass the test. | Not seen here yet; from the combined Ornith guidance (`~/projects/ornith/GUIDANCE.md` §5): crossbar's given tests pinned event positions and a tie-break no implementation following the task could meet. Listed in `docs/plans/CHECKLIST.md`. |
+| T42 | Before hand-over, grep every earlier given file and fixture for anything the task changes (strings, sentences, JSON shapes, counts). | T19 is one case of it (M3a `strict.rs`); crossbar had five such breaks (Ornith guidance §5). M5a task 22 handled it by keeping `edit.rs-task06` and `serve.rs-task10`. |
+| T43 | Run the gate's own checks on the given files themselves (`cargo fmt --check`, the line limit, clippy on test targets) before hand-over. A given file that fails the gate stalls the run, and the implementer may not edit it. | M4a task 16: a given test copied into the plan before `cargo fmt` came back reformatted (T29); crossbar stalled on given files that failed its gate (Ornith guidance §5). |
 
 ## What worked and should be kept