Reference doc mirrors edge-probe.md structure but links rather than re-argues;
states the MIXED-axis boundary (closed compiled shape-rooted subset here; open
UX subset - real-time/offline, a11y depth, i18n/RTL - prose-owned in
domain-probes.md). Docs-parity test pins doc taxonomy ids == code UI_TAXONOMY
ids and asserts disjointness from domain-probes.md topics (ADR-456
runtime-contract exemption, see #1867).