fix(task-2): headless block_set_attributes rewrites the embedded MTEXT; block_explode refuses members ezdxf would silently drop

Two review findings on the headless engine, both content lost with ok: True.

Multi-line attribute writer: `block_set_attributes` set only `attrib.dxf.text`,
so after an edit `block_get_attributes` reported the new value while
`block_explode` (`_burst_attrib`) burst the embedded MTEXT at its pre-edit
content -- and on the default R2010 document, where ezdxf exports no embedded
MTEXT, a save/reload carried the new value the exploded drawing never showed.
`_write_attrib_value` now rebuilds the embedded MTEXT from its own virtual
entity with the new content (`virtual_mtext_entity()` + `set_mtext(...,
graphic_properties=False)`, the round trip ezdxf's `BaseAttrib.transform`
uses), keeping placement, style and height; `_attrib_value` reports the
embedded MTEXT's full `Line1\PLine2` content for a multi-line attribute, as
AutoCAD's `TextString` does, so get / set / explode / COM agree.

Opaque members: `block_explode` iterated `virtual_entities()` without a
`skipped_entity_callback`, so ezdxf dropped every member it cannot copy
(OLE2FRAME) or transform (VIEWPORT) with a debug log, an ACAD_PROXY_ENTITY
without proxy graphics contributed nothing, and the reference was then deleted.
`_explodable_members` materializes the list before any write and refuses by
member with the typed capability `explode_opaque_members` (declared on both
engines: headless False with the reason, COM native), naming the block and
the live way out. The block_explode docstrings (backend and tool) now state
this refusal instead of claiming the xref/MINSERT gate covered everything.

Tests: eight new headless cases in tests/test_track_a_hardening.py -- the
multi-line reader, the rewrite (value on every surface, placement unchanged,
clean audit), a `\P` value and the empty value, an R2010 save/reload, the
single-line path unchanged, the opaque-member refusal (nothing written,
reference alive, members named), an ordinary block still exploding under a
uniform scale, and the capability declared on both engines.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

backends/com_backend.py

@@ -858,6 +858,9 @@ class ComBackend(AutoCADBackend):
                     False, reason="window_capture_has_no_render_to_label"
                 ),
                 "measure_area_acis": FeatureCapability(True, "native"),
+                "explode_opaque_members": FeatureCapability(
+                    True, "native", reason="autocad_explode;ole_and_proxy_members_unverified_live"
+                ),
                 "ocs_normalized": FeatureCapability(
                     True,
                     "activex_wcs",