DXF: write viewport frozen layers as group 341 layer handles too (#827)

A layout VIEWPORT's frozen layers were saved only as CADCraft's own
1001 CADCRAFT / 1000 VPFROZEN xdata. Other programs ignore that, so in
any third-party viewer every viewport showed every layer.

The writer now also emits the standard group 341: one soft pointer per
frozen layer to its LAYER table record handle, after 90/340 and before
the xdata. A frozen name with no LAYER record is skipped for 341 only
(the xdata still carries it). The reader keeps preferring the xdata; a
VIEWPORT without it (a file from another program) resolves its 341
handles through the LAYER records' group-5 handles, ignoring unknown
handles and keeping the 341 order.

Not covered: the DXF-to-DWG converter does not carry group 341 yet, so
a DWG keeps per-viewport freezing only through the CADCraft xdata.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Brandon Thomas <echelon@gmail.com>

docs/file-format-parity.md

@@ -1,6 +1,6 @@
 # File-format parity
 
-> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-12 · **Change:** minor (the Model layout's plot type is written as extents, not "Layout information") · **Target:** Autodesk AutoCAD 2027
+> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-12 · **Change:** minor (VIEWPORT frozen layers as group 341; code pages, #505; native MULTILEADER, #459; foreign data kept, #528; PDF text, layers and sheets, #567, #571; templates and user files, #552; plotter devices, #564) · **Target:** Autodesk AutoCAD 2027
 
 Every format AutoCAD 2027 reads or writes, with CADCraft's support, fidelity and tests. Work
 items are in [gaps.md](gaps.md#file-formats); the weighted score feeds
@@ -80,7 +80,7 @@ early (#175), but the throughput problem remains. **DWG ≈ 30% read, ≈ 15% wr
 | HATCH gradient | yes | yes (#162, fixes #80) | |
 | SOLID, TRACE, 3DFACE | yes | yes | |
 | ACAD_TABLE | yes | yes (entity + block) | |
-| VIEWPORT | yes | yes; frozen-layer lists kept (#150) | VP layer colour overrides lost (#99, PR #197) |
+| VIEWPORT | yes | yes; frozen-layer lists kept (#150), also as standard group 341 layer handles that other programs read | DWG: the DXF→DWG converter does not yet write group 341 (frozen layers survive a CADCraft round trip via xdata); VP layer colour overrides lost (#99, PR #197) |
 | IMAGE | yes, with IMAGEDEF | yes, with IMAGEDEF and IMAGEDEF_REACTOR | no attach command or display of the picture yet (FF10) |
 | WIPEOUT | yes | yes (#147, fixes #79) | |
 | MLINE (+ MLINESTYLE), HELIX, TOLERANCE | yes (#510, #420) | yes | |
@@ -135,6 +135,7 @@ our output in AutoCAD black-box.
 | Date | Change | Summary |
 |---|---|---|
 | 2026-10-12 | minor | DXF/DWG save: the Model layout's plot settings are written as CADCraft plots model space, drawing extents scaled to fit (PLOTSETTINGS 74 = 1, 75 = 0), instead of plot type 5 "Layout information", which other programs' Plot dialogs can't use for model space |
+| 2026-10-12 | minor | VIEWPORT frozen layers also written and read as AutoCAD's group 341 layer handles, so third-party viewers honour per-viewport freezing in DXF (DWG converter gap noted) |
 | 2026-10-11 | minor | Rows refreshed for merged work: code pages and `\M+` (#505), MULTILEADER (#459), IMAGE, MLINE/HELIX/REGION/TOLERANCE, VPORT/VIEW/UCS tables, modelled and raw objects, foreign data (#528), DWT templates and user `.lin`/`.pat`/`.pgp` files (#552), PDF output (#567, #571), plotter devices (#564), data extraction (#433, #534); weighted percentages not re-measured |
 | 2026-10-11 | minor | DXF (ASCII and binary) and DWG saved at 2000–2018, 2018 by default (#421) |
 | 2026-10-10 | minor | WIPEOUT and gradients written since #147/#162; other save losses remain (#201, #204, #208, #209, #213, #223) |