Fix DXF reader dropping AcDbPolyline subclass fields (LWPOLYLINE flags/elevation)

[kretz]

Fixes #5

Problem

LwPolyline has no entry in src/Metadata/LookupTables/Entities.ts, so DxfMap.create(LwPolyline) produced a map with no AcDbPolyline subclass. Every group code that _readLwPolyline delegates to tryAssignCurrentValue was therefore discarded silently:

• 70 — flags / isClosed
• 38 — elevation
• 39 — thickness
• 43 — constantWidth
• 210/220/230 — normal

Vertices parsed fine because the reader handles codes 10/20/40/41/42/50/91 explicitly.

Fix

Added the missing LwPolyline → AcDbPolyline metadata entry, matching the shape of neighbouring entries (e.g. Line, Circle). No reader/writer changes needed — the DXF writer already writes these codes explicitly, and the DWG reader decodes them from the bitstream directly, so both were unaffected.

Test

ReadLwPolylineSubclassFields in tests/IO/DXF/DxfReader.test.ts builds a minimal ASCII DXF with a closed LWPOLYLINE (70=1, 38=5, 39=2, 43=0.5) and asserts all four round-trip. It fails before this change and passes after.

tsc --noEmit clean, full suite: 463 passed / 6 skipped.

Related, not included

The same metadata gap exists for old-style POLYLINE: there are no entries for Polyline (the abstract base), Polyline2D or Polyline3D. Consequences:

• DxfClassMap.create(Polyline2D) / (Polyline3D) in _readPolylineCodes (DxfSectionReaderBase.ts:1098) throws "Polyline2D is not a DXF subclass", which failsafe swallows as an An error occurred while assigning a property using mapper notification — so the subclass map is never registered and the fields fall back to constructor defaults.
• PolyfaceMesh / PolygonMesh do have entries, but their base chain dead-ends at the missing Polyline entry, so they only pick up their own codes and lose the inherited Polyline ones (flags 70, elevation 30, thickness 39, default widths 40/41, normal 210).

Left out to keep this PR scoped to the reported issue — happy to follow up here or in a separate PR, whichever you prefer.

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