docs(spec,plan): track H states what it does with xrefs

The spec promised that the first live task would measure xrefs. It never
did: the plan measures model space, paper space and block definitions. 1.6
does not read an xref's own entities on either engine; the host drawing is
what gets read. The snapshot docstring and drawing_understand's docstring
now say so, and nothing is claimed about the live engine that was never
measured.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

docs/superpowers/plans/2026-09-24-v1.6-understand-implementation.md

@@ -1951,7 +1951,10 @@ Where the snapshot comes from:
   - **the selection set does not narrow a DXF export**: an empty selection set
     wrote the same model space, paper space and blocks as the set of all. The
     set of all is passed anyway, because the ActiveX signature requires one.
-  - xrefs were not measured.
+  - xrefs were not measured, so nothing is claimed for the live engine;
+    headless, ezdxf reads the host file only. Either way an xref's own
+    entities are not read in 1.6 - the host drawing is - and
+    ``drawing_understand``'s docstring says so.
 
   A DWG path on the live engine is opened read-only, exported the same way and
   closed without saving.
@@ -5506,6 +5509,7 @@ async def drawing_understand(
     and a confidence. Never modifies the drawing: a file is read as it is, and
     the current document through a snapshot (headless: the document in memory;
     live: a `Document.Export` DXF copy in a temporary folder, deleted after).
+    An xref's own entities are not read - the host drawing is.
 
     - `units`: `$INSUNITS` as declared against the unit the geometry implies
       (text heights, wall thickness, door swings, labelled room areas, overall