Task: Build the faithful document layer

(Describe what user-visible-or-internal change this task produces.)

(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.

The scanner and the document model are in product code, and the whole corpus goes through them: 1,623 documents read, written back, compared equal, and validating against input.xsd. The corpus found two silent data losses that no test written from the schema would have found, and both are now tests.

The layer needs no schema, so it landed before the schema tables rather than after them. That changes the order of the work the story had planned: the generator now serves the projection, not the round trip.

ctest has not run in this worktree. The tree predates the checkout's domain.hpp and the rebuild was still at 436 of 2,909 targets after an hour. The same sources are compiled and run directly against Catch2, which is where the results above come from. The CMake wiring is unproven until a build completes.