Task: Round trip the Basel traffic light configuration

The document is one struct, so it proves the thinnest possible path through the harness and is a template for the last file types.

• A file built from the schema imports, exports and matches, and the fixture is committed with the test.
• A bucket limit is a row keyed to the bucket, and the amber and red limits are typed columns.
• The missing corpus file is recorded as a measurement, with the command that found no file.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Every number this task claims about a tree, with the command that produced it and the commit it was measured at. A number without both is not a measurement, because a clean-up record describes a tree and trunk moves under a branch: a consumer count, a gate's component count and a suite's size all change with no code changing, and the reader of a bare number cannot tell which tree it came from. Where the raw output matters, put it in an evidence/ file beside the task and name that file in the Command column.

Manual QA scenarios (scaffolded via compass add test_scenario) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.