M3b plan: task 11 starts from a compiling skeleton of container.rs

Two sessions ended with nothing written, each out of room while planning the
whole file in one turn. The skeleton has the signatures, the constants and
the steps as comments; the task says to fill one function at a time.

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

docs/plans/M3b/README.md

@@ -69,6 +69,11 @@ At the end: `make gate` prints `gate: ok` with about 638 tests.
   `egress_dir` for task 12 but gave nothing that read it, so clippy's `dead_code` would fail the
   gate and `#[allow]` is forbidden: a task that could not be written as described (tip T16). The
   reference had a public `egress_dir()` getter; the task now gives it. Resume from task 11.
+- 2026-09-23, task 11 again: the second session also ended with nothing written, cut off while
+  deliberating over API details the compiler would have settled. The task now hands over a
+  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.
 
 ## Running it