DWG reader: fix PSTYLEMODE, hatch spline edges, proxy colors, MText and table robustness

[jarrett-meyer]

This PR fixes several issues found while reading real-world DWG files. Each fix is in its own commit.

PSTYLEMODE read as 8192

DwgHeaderReader assigned flags & 0x2000 directly to CadHeader.PlotStyleMode, so a drawing that uses color-dependent plot styles reported 8192 instead of 1. The DXF reader and the DWG writer both use 1/0, so the value is now mapped to 1/0.

Reproduce: create a CadDocument with PlotStyleMode = 1, write it as DWG (R2000 or later) and read the header back. Before this change the value read is 8192.

Hatch spline edges converted to closed splines with zero weights

Hatch.BoundaryPath.Spline.ToEntity():

• set IsClosed = true, but a spline edge is one open piece of the boundary loop;
• copied the Z of every control point as the weight. Non-rational edges can store 0 there, which gives a spline with all weights 0 that cannot be evaluated. Weights are now 1 for non-rational edges (and for non-positive values).

Proxy graphics true color and truncated proxy graphics

• In ProxyGeometryReader, the true color value is 0xmmRRGGBB stored little-endian, so the bytes come in blue, green, red order. Red and blue were swapped.
• A truncated or unexpected record in the proxy graphics threw and lost every geometry already read. Reading now stops at the bad record and keeps the geometries read so far.

MText dropped because of its background transparency

The MText background transparency (BL 441) is stored like an entity transparency: the type in the high byte and the alpha in the low byte. AutoCAD can leave stale bits in this field when the type is ByLayer. The raw value was passed to new Transparency(short), which throws for values outside 0-90, so the whole MText was dropped. Values from 0 to 90 are still used as before. Values with a type byte are decoded with Transparency.FromAlphaValue. Anything else falls back to ByLayer.

ACAD_TABLE dropped when its cell data cannot be parsed

The insert part of a table (position, scale and the anonymous block with its graphics) is read before the cell data. If the cell data could not be parsed, the whole entity was lost. The cell data is now read in a separate method. A failure there reports a warning and keeps the table with its graphics. CadTableComponentTemplate.Build now tolerates a missing cell style template from a partly read table.

Tests

• HatchTests.BoundaryPathSplineToEntityTest: checks the closed flag and the weights for non-rational and rational edges.
• DwgWriterTests.WriteHeaderPlotStyleModeTest: round-trips PlotStyleMode 0 and 1 through DWG for every supported version from R2000.

Both tests fail before these changes. The full ACadSharp.Tests suite passes on net9.0 (2408 passed, 2 skipped).

The proxy color, MText transparency and table changes have no synthetic test. Reproducing them needs DWG content that the writer cannot produce (AutoCAD proxy graphics, stale transparency bits, and table cell data the reader fails on). They were checked against real drawings that I cannot share.

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