fix(task-4): block_define dedupes requested layers case-insensitively so one layer in two spellings is created and reported once

DXF and ActiveX layer tables are case-insensitive, but block_define collected
the primitives' layers with a case-sensitive set. A request naming 'Alpha' and
'ALPHA' with create_layers=True made ezdxf add the first, then raise
DXFTableEntryError on the second after the write — layer left behind, no block,
surfaced as an internal error instead of a named refusal. COM ran Layers.Add
twice, got the existing layer back the second time, and reported two layers
created for one that appeared; the two engines diverged and the refusal message
without create_layers named the same layer twice.

referenced_layers() in backends/block_specs.py keys the set on the folded name
and keeps the first spelling; both engines consume it. _FakeLayers in the
hardening tests now matches Item/Add case-insensitively and returns the existing
record on a duplicate Add, as the ActiveX table does, so the COM path is pinned.

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

backends/ezdxf_backend.py

@@ -4993,6 +4993,7 @@ class EzdxfBackend(AutoCADBackend):
         from ezdxf.enums import TextEntityAlignment
 
         from backends.block_specs import (
+            referenced_layers,
             solid_vertices,
             validate_attdef_specs,
             validate_base_point,
@@ -5009,7 +5010,7 @@ class EzdxfBackend(AutoCADBackend):
         ents = validate_entity_specs(entities)
         atts = validate_attdef_specs(attdefs or [])
         base = (*validate_base_point(base_x, base_y), 0.0)
-        wanted_layers = sorted({spec["layer"] for spec in ents if spec["layer"] != "0"})
+        wanted_layers = referenced_layers(ents)
         align_map = {
             "left": TextEntityAlignment.LEFT,
             "center": TextEntityAlignment.CENTER,
@@ -5026,6 +5027,7 @@ class EzdxfBackend(AutoCADBackend):
                     "pass overwrite=true to replace its contents"
                 )
             replaced = existing is not None
+            # ``in doc.layers`` is case-insensitive, like the table itself.
             missing_layers = [layer for layer in wanted_layers if layer not in doc.layers]
             if missing_layers and not create_layers:
                 raise ValueError(