[mahach666]

What changes and why

Fixes #7. The full-width form (tag 00) of packed shorts and packed integers was read with ReadBits(16) / ReadBits(32), which puts the first byte in the most significant position. These values are stored little-endian. Entity records that hold a colour, extended data size or reactor count in that form were then decoded from the wrong bit position. Such records failed, or decoded with NaN, infinite or absurd coordinates. Some JSON exports failed on the NaN values.

ReadPackedShort and ReadPackedInt now read the full-width form byte by byte, low byte first, the same way ReadRawUInt32 reads raw integers. The short forms (tags 01, 10, 11) are unchanged. The object type of pre-R2010 records goes through ReadPackedShort and is corrected as well.

Tests added in BitReaderTests:

• full-width forms, aligned and unaligned;
• the unchanged short forms;
• the object type of pre-R2010 records.

Four of the new tests fail on the old reader. The short-form test passes on both.

How it was checked

• dotnet build DwgIO.slnx -c Release has no warnings
• dotnet test passes: 69 passed and 7 skipped (fixture tests) on net8.0 and net10.0. With private drawings, the same 7 fixture tests fail as on main, for already known reasons: AppInfo is read as two fixed entries, so one-entry drawings fail and Version stays empty #11, AcDb:Classes is rejected when its declared size is a few bytes larger than its pages hold #12 and Fixture tests fail when DWGIO_TEST_FILES contains drawings of an unsupported version #17.
• JSON exports and dwgio inspect output of the same drawings before and after are identical, or the differences are listed and intended:
  • Every output that is not an entity is identical before and after, across all AC1024, AC1027 and AC1032 drawings checked. This covers sections, the container header, the handle map, the object inventory, metadata, the preview and the section table of inspect.
  • Entity records with a diagnostic drop from 24% to 0.24%. Every remaining one is a DICTIONARY record (DICTIONARY objects (type 0x2A) are decoded as entities and listed in DwgDocument.Entities #8).
  • Entities with NaN, infinite or absurd coordinates drop to none, and JSON exports that used to fail on non-finite values now succeed.
  • No drawing has fewer decoded entities or more diagnostics than before.
• The change is original work; no code or text is copied from other projects
• No drawings, customer data, vendor code or binaries are added

Notes for review:

• AC1018 is covered by unit tests only; no AC1018 drawing was available.
• With correct values the colour index problem becomes visible. It is tracked in Entity colour index keeps only its low byte, so BYLAYER (256) is reported as 0 #40 and is not changed here.