M4a tasks 08 and 09: finer skeletons after task 08's session wrote nothing

Task 08's `header` was one todo!() with a dozen branches; Ornith planned it in its head until the
turn ran out (tip T25). `next_message` and `header` are now written as glue over seven small
helpers, and task 09's `poll` over four, each checked to pass when filled from its comments.

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

docs/plans/M4a/README.md

@@ -65,6 +65,20 @@ checked to compile against its task's tests and fail them.
 
 At the end of task 15: about 762 tests (650 before task 01).
 
+## Changes during the run
+
+- 2026-09-23, task 08: tasks 01 to 07 committed; task 08's session ended with nothing written. It
+  planned all four `todo!()`s in its head, most of it deliberating how to avoid indexing, and was
+  cut off (tip T25, again: `Decoder::header` has a dozen branches and was one `todo!()`). The
+  tree also had `handshake.rs` with its imports in 2021-edition order, which no logged command
+  explains; it was reverted. The attempt is saved in `.state/runs/M4a/08-first-attempt.diff`.
+  The design model split the skeleton: `next_message` and `header` are now written as glue, over
+  seven helpers of a few lines each whose comments give the exact expressions. The same was done
+  for task 09's `poll` (glue over four helpers) before it could stop the same way. Both new
+  skeletons were checked in a scratch copy of the branch: they compile and fail their tests, and
+  filled literally from their comments they pass (7 of 7, 10 of 10 five times), clippy clean, gate
+  ok. Resume from task 08.
+
 ## Running it
 
 ```sh