[Sring65]

Note: All modifications are AI-generated code.

Summary

A set of fixes so that DWG/DXF files read/written by ACadSharp open cleanly in
AutoCAD and round-trip (DWG → DWG / DXF) with correct geometry, associations,
and content. The changes are based on the current upstream line (v3.8.16).

This is a fairly large, multi-topic PR. I've grouped the fixes below so they can
be reviewed theme-by-theme.

Dependency: DomCR/CSUtilities

This PR depends on a companion change in DomCR/CSUtilities — a fix to the
Matrix3(Matrix4) constructor. Without it, the transform fixes in this PR
(ellipses, text rotation, arc angles) come out mirrored. The src/CSUtilities
submodule is pointed at that commit.

• CSUtilities PR:
• Submodule commit: fbf0723

Please merge the CSUtilities change first, or let me know if you'd prefer I
re-point the submodule.

What's fixed

R2007 (AC1021) DWG write

• Implemented AC1021 output.
• Page-record Checksum is computed over decompressed data and CRC over the
  compressed stored data (previously both over compressed data → "Error
  decrypting data").
• enc=1 sections (SummaryInfo/Preview/AppInfo/AppInfoHistory/FileDepList) stored
  uncompressed for AC1021.
• Encoded page physical size now accounts for the parity region instead of a fixed
  maxPageSize + 32, so large preview data no longer overflows and shifts pages.

Tables (AcDbTable)

• DXF read of inlined cell contents now marks ContentType so R2010+ doesn't drop
  the whole table (Error 34 eWrongObjectType).
• R2004 table write: 4th override bit / object-stream alignment.
• R2010+ cached block reference restored (table not displaying).
• Pre-R2007/R2007 legacy table-level override section preserved on write.

Dimensions & DIMASSOC

• Dimension association read field order corrected to match the ODA spec
  (associativity / trans_space / rotated_type / dimension object) — was reversed,
  so native files' associations came out misaligned.
• Dimension geometry follows transforms; OsnapPointRef last-point-ref bit kept.

SHAPE

• DXF/DWG group code 2 writes the shape name (not the style name) so AutoCAD
  doesn't drop it.
• Added shape-file index and search next to the drawing; fixes AutoCAD AUDIT
  "shape number/index <= 0".

Coordinate / transform (ApplyTransform)

• Fixed for Arc, Shape, Text, Underlay, MLine, Point, Hatch, Leader, Dimension
  series, Spline, TableEntity; Insert scaling/rotation; Ellipse axis vectors +
  normal (with the CSUtilities Matrix3 transpose fix).

XDATA / string encoding

• Pre-R2004 Chinese XDATA no longer garbled.
• TV/TU strings now null-terminated (fixes class-name corruption).

Round-trip fidelity

• Hatch origin/seed points, LWPOLYLINE width/const-width (43 flag), Leader
  hook line/text width/association, PDFUnderlay scale/rotation, Text/Attrib/Shape
  default omission, Wipeout clip close, HatchPattern angle, Spline nodes.

Gradient fill

• R2000 gradient definition parse, R2004+ → R2000 down-conversion, R/B channel
  order, LWDISPLAY header variable.

Object stream / handle alignment

• ACDS dangling reference (file corruption), PolyFaceMesh owned handles
  (HardOwnership), UCS/VIEW control-object handle slots, table-style soft pointer.

Verification

• Affected files open in AutoCAD without errors (and pass ODA FileConverter audit).
• Round-trip output compared against AutoCAD-native files: entity counts,
  geometry, associations, and content match.
• Where a defect existed, the produced file was diffed field-by-field against the
  AutoCAD reference.

Notes for reviewer

• Please review theme-by-theme using the headings above.
• R2007 output is new functionality — worth a closer look on the write path.
• Happy to split this into separate PRs per theme if that's easier to review.