[dsn27]

Part of #412 (step 6). Stacked on #472; retarget once the font stack is merged.

Why

With #468, texts on Windows use the managed font reader instead of GDI+. GDI+ reports every style of every family as available, and synthesizes the ones a family does not have. So before the font service, a bold or italic text in a family without that face was shown synthetic bold or italic. The managed provider took the closest face instead, which would have dropped the style.

Families without a bold face on a Windows runner include Microsoft Sans Serif (the default font), Lucida Console, Lucida Sans Unicode, Impact, Sylfaen, Gabriola, MS Gothic, Segoe UI Light and Segoe UI Symbol. This PR keeps those texts as they were.

What

ManagedFontProvider.GetFace still picks the closest face. If bold or italic is still missing, it wraps that face in the new internal SyntheticFontFace.

Style	What GDI+ does, measured on Windows	Implementation
italic	shear by 0x5700/0x10000 (18.77°) around the baseline; advance unchanged; identical in GDI and GDI+ with no residual	the same shear
bold	the left side and the baseline stay; every edge facing right moves right, every edge facing up moves up; advance unchanged	every point moves along the bisector of its edges as in FreeType's FT_Outline_EmboldenXY, by half the strength, then everything shifts by half the strength to the right and up. Like GDI+, and unlike FreeType, the shift of each point is kept within half the strength on each axis instead of being limited by the length of the shorter edge at inner corners. TrueType glyphs are widened on their quadratic outline before the conversion to cubic curves
bold strength	⌈em/50⌉ font units horizontally and one unit less vertically: 41/40 for 2048, 82/81 for 4096, 21/20 for 1024, 20/19 for 1000, 6/5 for 256; independent of the size requested from AddString	the same rule
bold on a face of weight ≥ 600	no widening (semibold, black)	the same; TrueTypeFont.WeightClass reads usWeightClass, or the wght axis of a variable instance

The advance widths stay unchanged, as with the GDI widths that CADability used for its texts.

The measurements come from a diagnostics branch, diag/synthetic-styles, which also compared the outlines point by point with GDI+. It is not for merging and can be deleted.

Tests

SyntheticFontFaceTests, on every platform:

• The emboldened outlines of Lucida Console "I" (12 points, with inner edges) and Microsoft Sans Serif "I" equal the outlines GDI+ returned on Windows, to 0.01 font units.
• The hole of a glyph gets smaller, because the orientation is taken over the whole outline as in FreeType.
• A short edge at an inner corner moves by the whole strength, and the tip of a thin spike moves no further than the strength on either axis.
• The strength rule holds for 256 to 4096 units per em, and a bold face is not widened again.
• On the regular-only Liberation Sans, the italic, bold and bold italic faces are synthesized, made once, and keep the metrics and advance widths. Italic is the exact shear; bold keeps the left side and baseline and grows by 41/40 units.

SyntheticStyleGdiTests, Windows only:

• It compares the change each synthetic style makes to the regular outline with the change GDI+ makes, for the glyphs "HIlo8". The points are matched by position, because GDI+ starts some contours (MS Gothic) at another point. When GDI+ splits a curve differently, the test compares the change of the bounding box instead.
• Microsoft Sans Serif, Lucida Console, Lucida Sans Unicode, Impact and MS Gothic: tolerance 2 font units.
• Sylfaen and Gabriola: tolerance 2% of the em height, see below.

GdiFontComparisonTests now compares only real faces; synthetic faces are covered by the test above.

CADability.Tests on Linux: 685 passed, 0 failed.

Not the same as GDI+

• In Sylfaen and Gabriola, GDI+ moves single points differently, and no rule for it could be found. For example, a point in the middle of a straight, slanted edge of the Sylfaen "8" only moves up. The largest differences are 1.6% (Sylfaen "8") and 0.85% (Gabriola "H") of the em height, about 0.2 pt for 12 pt text.
• For named instances of some variable fonts (Bahnschrift SemiLight, the Segoe UI Variable Light instances), GDI+ widens a different instance, so its result cannot be reproduced from the instance itself. Those cases keep the rule above.
• Underline and strikeout bars are transformed together with the glyph.

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Generated by Claude Code