M5a plan, task 04: chat-template tags built from pieces

Task 04's session read the given test and the check.rs skeleton, both
holding the tags the check refuses, and Ornith took them for its own
template. It lost track of the file and made no change. The tags are
now built with concat! and format! in check.rs and in the four given
tests that held them, and the spec describes them in words. The
rewritten tests pass against the reference, are rustfmt-clean, and
still catch the case-folding mutation.

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

docs/plans/M5a/04-memoryd-entries-check.md

@@ -32,6 +32,11 @@ Two small modules (M5a spec, section 3 and section 5, "The check"):
 `DEFAULT_IGNORABLE`, `ALSO_INVISIBLE`, `MARKUP` and `PHRASES` are written; fill `invisible` and
 `check`. Do not change the tables.
 
+**Markup in this code.** `MARKUP` and the test build the chat-template tags from pieces
+(`concat!("<", "think")`, `format!("{LT}think>")`), so that no file holds a tag literally. Keep it that
+way: do not write a tag out whole in code, a comment, a command or your log. When you need to
+name one, say "the opening think tag".
+
 ## Steps
 
 - [ ] **1. Copy** the four files; edit `lib.rs`.