docs(playbook): re-read comments inside your own diff; check modals for state of their own

A commit that removed a modal's self-closing behaviour shipped two comments in the
same file still describing it as self-closing — review checks whether the code is
right, not whether the surrounding prose survived. And the property that predicts
which modals break on refresh is 'holds no state of its own' (pure projection of a
list), not the presence of a detail field.

admin_ui_cleanup.md

@@ -447,6 +447,23 @@ A proof about the container is not a proof about the member, and that substituti
 because the evidence is real and the reasoning feels airtight. Ask explicitly: *does my evidence
 range over exactly the thing I am ruling out, or over something that contains it?*
 
+**Re-read the comments inside your own diff — a change that invalidates prose usually ships the
+prose.** A commit that removed a modal's self-closing behaviour left two comments in the same file,
+in the same commit, still describing the modal as self-closing. The code and its explanation
+contradicted each other inside one change, and the diff read as coherent because a reviewer checks
+whether the *code* is right, not whether the surrounding sentences survived it. Grep your own diff
+for comments in the touched regions and ask what each one asserts. This is the most common way a
+correct fix ships a false claim, and it is invisible afterwards: the next reader trusts the comment.
+
+**When a modal misbehaves on refresh, the property to check is whether it holds any state of its
+own.** A modal that is a *pure projection of a list* cannot distinguish "paged out of view" from
+"deleted", so any list change is indistinguishable from the record vanishing. That — not the presence
+of a particular field — is the test. In one sweep, four detail surfaces split exactly on it: the two
+that fetch detail by id survive a page change with usable content, the two that project from the list
+are page-scoped whether or not anyone intended them to be, and only the latter two need their held id
+cleared on paging. Checking for the *field* would have found the same two here by luck; checking for
+*state of its own* is what generalises.
+
 **A "what I could not establish" section confers unearned credibility on everything above it.**
 Listing your limitations is good practice and this playbook demands it — but be aware of what it does
 to a reader: the presence of a limitations section is taken as evidence that the rest was held to