Record the resolution of the M3a "fsync stall": a macOS socket rule

The handoff and the two stopped rows blamed an environment fsync stall.
The cause was macOS refusing socket options after the peer closes, in
loopd and in the brokerd test client. The handoff now opens with the
resolution, the log gets a review note (the stopped rows are kept as
written), and the lessons gain I13 (measure what you blame, and name
the machine) and T18 (the gate must pass on Talos and on the Mac).

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

docs/implementer-lessons.md

@@ -28,6 +28,7 @@ How it is used:
 | I10 | If a tool you were told to use does not exist, stop and say so. Do not invent a command in its place. | Laguna as coordinator ran `opencodec`, which does not exist, and then diagnosed its own typo. | yes | |
 | I11 | A rule about one path applies to every path that does the same thing. If a task says "release X before the final frame", every final frame counts, including the error frames written earlier in the function. | M2b finding 1. | yes | |
 | I12 | A file that exists but cannot be read is an error. Only a file that does not exist may be treated as absent. | M2b finding 3. | yes | |
+| I13 | Before calling a failure "environmental", measure the thing you blame, and name the machine, OS and file system the evidence came from. A sample that shows a thread inside a system call shows where the time goes, not that the call is stuck. | M3a stop before task 20: an fsync "stall" was blamed from `__fcntl` frames that were ordinary 4 ms `F_FULLFSYNC` calls on the Mac, and the real cause, a macOS socket rule, went unfixed. | no | |
 
 ## Tips for writing tasks
 
@@ -50,6 +51,7 @@ How it is used:
 | T15 | Running the whole plan through `tools/run-plan.sh` worked: ten tasks, one commit each, unattended, in about three hours. Keep the TUI closed while it runs; a second message into the driver's session starts a second agent on the same tree. | M2b run, 2026-09-18. |
 | T16 | When a task prescribes the fix, check that it compiles against the types as they are, in the reference tree, before handing it over. A fix that cannot be written as described pushes the implementer outside the listed paths. | M2b task 11: "`unwrap_or_else` with a fixed valid id" had no non-panicking form outside `proto`; the implementer added `Default` to `SessionId` and reported it. |
 | T17 | Match the check to the risk. A full reference for intricate logic whose writing debugs the spec (state machines, concurrency); a naive oracle inside the test for decision logic; a compiling skeleton (`todo!()` bodies under the real signatures) for plumbing. Record what each check exposed, and let the record decide what the next milestone gets. | Across M1 to M2b the references caught no implementer defect. They caught task defects (T16) and missed what they shared with the tests (T5). Decision of 2026-09-18. |
+| T18 | The gate runs on two platforms, Talos (Linux) and the Mac (macOS), and they differ where the tests touch the OS: sockets, file sync, `/tmp`. Accept a task only when the gate passes on both. Code that sets a socket option after the peer may have closed breaks on macOS only. | M3a: `BrokerPort`, the HTTP client and the brokerd test client passed on Talos and failed on the Mac from task 13 on; it was found only when the gate was first run on the Mac. |
 
 ## What worked and should be kept