Task: Remove xsdcpp

Delete the generated domain, the sibling-checkout wiring, the two post-regeneration patches and the knowledge page that records the pin. Last, because nothing may depend on the generated binding by then.

• No code includes the generated domain header.
• The build no longer needs the sibling checkout.
• reapply_export_macros.py and reapply_ore_lexical_form.py are gone.
• The full gate passes with the generated files deleted.

(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 generated domain, the sibling-checkout wiring, the two regeneration patches, the golden dataset and the knowledge page are deleted. PR #3565 did this together with the conventions move. No product file includes the generated domain. The XML suites, the family census and the drift check pass.