M3b plan: task 11's skeleton writes run as glue over small helpers

The third attempt filled five functions, then ran out of room planning all of
run in one turn. run and run_container are now given; spawn, Io::start,
Io::finish and answer are small todo!()s. Checked fillable: 11 of 11 passed.

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

docs/plans/M3b/README.md

@@ -74,6 +74,13 @@ At the end: `make gate` prints `gate: ok` with about 638 tests.
   compiling skeleton of `container.rs` (signatures, constants, the steps as comments, `todo!()`
   bodies), checked against the given tests (11 red), and says to fill one function at a time with
   `cargo check` between. Resume from task 11.
+- 2026-09-23, task 11, third attempt: it filled five of the six functions from the skeleton, with
+  `cargo check` between, then planned all of `run` in one turn and was cut off (two compile errors
+  left). Saved in `.state/runs/M3b/11-third-attempt.diff`. The skeleton now has `run` and
+  `run_container` written as glue, and the rest as small helpers (`spawn`, `Io::start`,
+  `Io::finish`, `answer`); the design model filled them in a scratch copy to check the split can
+  pass (11 of 11, five runs, clippy clean) and removed that. Task 12 now calls `run_container`.
+  Resume from task 11.
 
 ## Running it