M4a task 16 review: accepted; the plan's copy of its test is now the formatted file

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

docs/implementer-log.md

@@ -570,3 +570,18 @@ T25); task 14 made the model read the whole crate to learn its calls (T25, a cal
 14's second session found `now_ms`'s comment on `post`, put there by the design model's script (tip
 T29). It saw the contradiction and would not guess, which was right, but it deliberated instead of
 stopping.
+
+### M4a, task 16 — reviewed 2026-09-24 by the design model (Claude)
+
+Accepted; finding 2 is closed. `read_checked` follows the task step for step: the path is checked
+without following links, the opened file must be the same one (device and inode), the owner and
+mode checks run on the open file, and the bytes come from that handle into `Zeroizing`. The gate
+passes; both of the plan's mutations (no inode check; the mode taken from the path) turn the
+given test red against this code. Against the owner's server, a token in a 0600 file gave the
+warning and connected, and the same file at 0644 was refused with its pointer; the token was in no
+output.
+
+The one deviation, reported, was the plan's fault: step 6 of the comment said "from the old
+`check_file`", which made step 5's `grep` print a line the task said it would not. The given test
+also differed from the plan's copy, in rustfmt's layout only: the design model copied it into the
+plan before formatting it. The plan's copy is now the formatted file.