fix(task-14): seed AutoCAD's default scale list before adding an annotation scale headlessly

An ezdxf-authored drawing has no ACAD_SCALELIST entries, and
`_ensure_scale_entry` appended only the requested scale. AutoCAD repopulates
its default list only when the dictionary is *empty*, so the saved file
opened with a one-entry scale list and `SetVariable("CANNOSCALE", "1:1")`
failed with `Error setting system variable` -- the annotation scale could
no longer be changed in AutoCAD, while the tool reported `ok: True` and the
test pinned the degraded state (`len(keys) == 1`).

An empty list is now seeded with AutoCAD's own default metric list before
the requested scale is considered: the 17 entries a fresh AutoCAD 2026
drawing carries (`Documents.Add()` saved as DXF, MEASUREMENT 1), in that
order and under AutoCAD's own keys A0 ... A9, B0 ... B6. New entries take
the next key in that sequence (`_scale_list_key`) instead of being keyed by
their name; presence is still decided by group 300, so a non-empty
AutoCAD-authored list is never re-seeded or duplicated. Verified live: the
headless file now opens in AutoCAD 2026 with the 17 keys, CANNOSCALE 1:50
current, and switching to 1:1 and 1:100 works.

Tests: the fresh-drawing test asserts the seeded names and keys, that
setting 1:50 appends nothing (it is a default), that 1:25 lands under B7
once, and that the list survives save + reopen with 1:1 present; a new test
pins that *reading* the annotation scale creates nothing (the read side
stays pure, AutoCAD fills an empty list itself); the AutoCAD-keyed list test
now expects the A3 key. The `drawing_settings` docstring says what each
engine does and that AutoCAD refuses the live write on a paper-space layout
with no active viewport (measured).

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

server.py

@@ -5082,7 +5082,9 @@ async def drawing_settings(
     headless engine (`capability: registry_sysvar` — AutoCAD keeps them in the
     registry, a file cannot); `dimstyle` / `textstyle` when the backend has no
     styles contract. On the live engine `annotation_scale` must name a scale in
-    the drawing's scale list (SCALELISTEDIT); the headless engine adds it.
+    the drawing's scale list (SCALELISTEDIT) and AutoCAD refuses the write on a
+    paper-space layout with no active viewport; the headless engine adds the
+    scale, seeding AutoCAD's default list first when the drawing has none.
     Headlessly, grid/snap are stored on the active VPORT and the annotation
     scale in the variable dictionary — the places AutoCAD reads them from.
     """