Diameter dimensions: the *D block's text keeps the sign as %%c (fixes #1645) (#1696)

The *D block and EXPLODE's MTEXT took the dimension's decoded text, so
%%c arrived as U+2300, which no SHX font has: other programs plotted a
diameter dimension in an SHX style without its sign. They now write
DimGeometry::mtext_contents(): the sign as %%c, which every font draws
(and CADCraft decodes), a % that would run into a code as %%%.

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:** 2026-10-19 · **Change:** minor (a file's own standard-named arrowhead blocks are kept) · **Target:** Autodesk AutoCAD 2027
+> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-20 · **Change:** minor (a diameter dimension's `*D` text keeps the sign as `%%c`) · **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
@@ -77,6 +77,7 @@ on read with a note, so such files no longer exceed the DXF limit (#1482). **DWG
 | POLYLINE 2D/3D | yes | yes (as LWPOLYLINE / 3D POLYLINE) | polyface and polygon meshes flattened: lossy |
 | TEXT, MTEXT, ATTDEF, INSERT + ATTRIB | yes; multiline attributes (embedded MTEXT) read and drawn | yes; multiline attributes written as embedded MTEXT (DXF 2007 and later; older versions: one line) | ATTDEF prompt kept (#137) |
 | DIMENSION (all 7 kinds) | yes | yes, with anonymous `*D` blocks | a reopened dimension's `*D` block is regenerated under its name, unreferenced ones dropped (#495); DIMASSOC objects written; angular block text honors DIMDSEP, including per-dimension overrides (#1646) |
+| DIMENSION (all 7 kinds) | yes | yes, with anonymous `*D` blocks; their text keeps the diameter sign as `%%c`, so SHX styles plot it (#1645) | a reopened dimension's `*D` block is regenerated under its name, unreferenced ones dropped (#495); DIMASSOC objects written |
 | LEADER | yes | yes | |
 | MULTILEADER | yes (#459) | yes, native MULTILEADER with MLEADERSTYLE (#459) | several landings per multileader are folded into one; per-object overrides and annotative scale contexts not kept |
 | HATCH pattern / solid | yes | yes; associative hatches keep their boundary objects in blocks too, and the boundary carries the hatch reactor (stretched dynamic blocks drag the fill in AutoCAD) | |
@@ -140,6 +141,7 @@ our output in AutoCAD black-box.
 |---|---|---|
 | 2026-10-19 | minor | DXF/DWG save: angular dimension block text honors DIMDSEP, including per-dimension overrides (#1646) |
 | 2026-10-19 | minor | A file's own arrowhead blocks with standard names (AutoCAD's `_Dot`, `_DotSmall`), named only by a dimension style, are kept and saved as they were instead of being replaced by CADCraft's solids; PURGE keeps arrowhead blocks dimension styles name |
+| 2026-10-20 | minor | DIMENSION: the `*D` block's MTEXT, and the MTEXT EXPLODE leaves, hold the diameter sign as `%%c` instead of the decoded U+2300, which no SHX font has, so other programs plotted diameter dimensions in an SHX style without the sign (#1645) |
 | 2026-10-17 | minor | DWG read: DGN line style components no line style definition reaches are left out before the DXF bridge, with a read note; an 83 MB site plan with 888,483 of them (1.2 GB of DXF, over the limit) now opens (#1482) |
 | 2026-10-12 | minor | PDF output: object transparency plotted as PDF alpha (`/ExtGState` `/ca` `/CA`) when the page setup's or the plot's "Plot transparency" is on; PNG plots blend it, SVG uses `opacity`; plots without it are unchanged byte for byte |
 | 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 |