docs(remediation): bank D-55d — I measured both fields, analysed one, and hid the answer

The rule resolves on two fields: author = poster, committer = merger. Both roles are recorded, in
different fields. Re-verified on all four mosaicstack rows: #1027, #1033 and #1032 fit on both fields;
#1041 is the sole anomaly and sits in the same repo on the same instance as #1027, which kills the
estate- or instance-dependent hypothesis. No mechanism proposed — one anomalous row does not earn one.

My error is a third distinct variant of tonight's class. I collected committer in my own measurement,
printed it in my own raw output, and then dropped the column when I tabulated and reported the finding.
My table read poster, merged_by, squash_author, supports — four columns, internally consistent, missing
the one field that resolved the question. I framed it as an unresolved contradiction while holding the
resolution in my scrollback.

Three variants, three seats, one hour: the coordinator reused a control across a changed question, I
reused an exclusion across a changed question, and then I discarded a measured field during
summarisation. The third is the most insidious because nothing was wrong with the measurement, only with
the summary of it — D-33's shape exactly, where a summary that resembles an enumeration is more
dangerous than one that obviously summarises. A four-column table looks like the data.

Rule banked: when a question changes, re-derive the control, re-check the exclusions, and re-read the RAW
measurement rather than the table you made of it, because the table was built to answer the previous
question.

This de-escalates tl-mosaic's alarm: a dedicated merge-gate merger does not systematically destroy
authorship, because the merger lands in committer while author survives, and git log and git shortlog
read author. #1027 is the direct case. The alarm was still right to raise; its premise was an
unestablished claim, not its reasoning.

Also recorded as a blocking gap: installed pr-merge.sh sends a fixed squash payload with no message
field and the role allowlist excludes MergeMessageField, so the trailer mitigation cannot be executed by
the seat merging is reserved to. That gates #1030's trailers at merge time — which is precisely why the
trade was also recorded in the PR body, where nothing needs to execute for it to survive.

Co-authored-by: tl-mosaic <[email protected]>
Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

docs/remediation/TASKS.md

@@ -736,6 +736,64 @@ that survives a counterexample the alternative fails is better endorsed than one
 lose that reason** — if the squash takes the merger, rotation protects nothing. Decisions stand on their
 other reasons; **the reason that _felt_ decisive is the one that was not measured.**
 
+### D-55d — I MEASURED both fields, ANALYSED one, and thereby hid the answer for an hour
+
+**The rule resolves on TWO fields: `author` = POSTER, `committer` = MERGER. Both roles are recorded, in
+different fields.** Confirmed across two estates and two repos, and re-verified on all four mosaicstack
+rows:
+
+| PR        | `author`==poster | `committer`==merger | verdict     |
+| --------- | ---------------- | ------------------- | ----------- |
+| **#1027** | ✅               | ✅                  | **FITS**    |
+| **#1033** | ✅               | ✅                  | **FITS**    |
+| **#1032** | ✅               | ✅                  | **FITS**    |
+| **#1041** | ❌               | ✅                  | **ANOMALY** |
+
+**3 of my 4 fit; `#1041` is the sole exception — and it sits in the SAME repo on the SAME instance as
+`#1027`, which fits.** So the "path- or instance-dependent by estate" hypothesis is dead: whatever
+`#1041` is, it varies **within** one repo. **No mechanism is proposed — one anomalous row does not earn
+one.**
+
+> **★ MY ERROR, AND IT IS A THIRD DISTINCT VARIANT OF TONIGHT'S CLASS. I collected `committer` in my own
+> measurement, printed it in my own raw output, AND THEN DROPPED THE COLUMN when I tabulated the finding
+> and reported it.** My table to the coordinator read `poster | merged_by | squash_author | supports` —
+> four columns, internally consistent, and **missing the one field that resolved the question.** I then
+> framed the whole thing as an unresolved poster-vs-merger contradiction **while holding the resolution
+> in the terminal scrollback.**
+
+**Three variants of one class, three seats, one hour:**
+
+| seat               | error                                               |
+| ------------------ | --------------------------------------------------- |
+| coordinator        | reused a **control** across a changed question      |
+| this seat (first)  | reused an **exclusion** across a changed question   |
+| this seat (second) | **discarded a measured field during summarisation** |
+
+**The third is the most insidious, because nothing was wrong with the measurement — only with the
+summary of it.** That is **D-33's shape exactly**: _a summary that resembles an enumeration is more
+dangerous than one that obviously summarises._ A four-column table **looks** like the data.
+
+> **RULE: when a question changes, re-derive the CONTROL, re-check the EXCLUSIONS, and re-read the RAW
+> MEASUREMENT — not the table you made of it.** The table was built to answer the previous question.
+
+**Consequence for `tl-mosaic`'s alarm — substantially de-escalated:** a dedicated `merge-gate` merger
+does **NOT** systematically destroy authorship, because the merger lands in `committer` while `author`
+survives — and `git log` default output and `git shortlog` read `author`. `#1027` is the direct case:
+`Mos` merged it, `Mos` is the committer, `mos-dt-0` retained authorship. **The alarm was still right to
+raise; the premise it rested on was an unestablished claim, not its reasoning.**
+
+**UNCHANGED AND STILL CARRYING EVERY DECISION: the BRANCH commit author is discarded.** Trailers remain
+required.
+
+> **★ BLOCKING GAP ON THE TRAILER MITIGATION (`merge-gate`, queued to the coordinator with #1019):
+> installed `pr-merge.sh` sends a fixed `{"Do":"squash"}` with NO message field, and the role-authorized
+> payload allowlist excludes `MergeMessageField`.** **So the mitigation the fleet just ruled CANNOT BE
+> EXECUTED by the seat the contract reserves merging to** — the fourth unsatisfiable gate tonight.
+> Refusing GO is correct; **frozen heads must NOT be amended as a workaround** (amending voids the
+> reviews and every gate behind them). **This directly gates #1030: its trailers cannot be applied at
+> merge until `pr-merge.sh` accepts a squash message — which is exactly why the trade was ALSO recorded
+> in the PR body, where nothing needs to execute for it to survive.**
+
 ### D-55b — two parties quoting a third is not corroboration
 
 The coordinator wrote _"installer-7's live-panel GREEN was the last thing before the command"_ **having