feat: ctxDrawSymbol and symbolSize, SF Symbols drawn in a surface in the fill colour

[sidorares]

For sidorares/react-x11#591: the system's icons by name, drawn into a surface for a renderer's own content. createStatusItem's image and a menu item's iconName already take an SF Symbol name; nothing that draws inside a window did, so an app whose tray icon is waveform could not show waveform in its own UI.

Rendered by this branch: the eight symbols the Hush port uses, at weights 300, 500 and 800 in three fill colours, and speaker.wave.3.fill at variableValue 0, 0.34, 0.67 and 1.

The verbs

• native.ctxDrawSymbol(surface, name, x, y, width, height, options?) draws the symbol fitted into the rect, centred and keeping its proportions, in the current fill colour, and answers true; for a name the catalogue does not know it answers false and draws nothing. A symbol is a template, its shape in whatever colour it is drawn with, so it is drawn the way ctxDrawGlyphs draws a run: the template goes into a transparency layer and the fill colour through it with SourceIn, and the layer composites once through the surface's CTM and clip, at its global alpha and blend mode, over what is already drawn.
• native.symbolSize(name, options?) answers { width, height } in points, the symbol's designed padding included, or null for an unknown name.

Both take pointSize, default 13; weight, 100 to 900, mapped to AppKit's weights the way matchFont maps them, a mapping now shared as FontWeightFromCss; scale, 'small', 'medium' or 'large'; and variableValue, 0 to 1, on macOS 13 and later. A name that is not a string is a TypeError.

In threaded mode both run on the calling thread, like every other ctx* verb and the font verbs: an NSImage drawn into a bitmap context is not a view, so it does not need the UI thread that bezels do. The test checks that a worker draws the same pixels as the main thread.

Tests

test/symbols.js, added to the test script and to CI:

• symbolSize: a size that grows with the point size and the weight; null for an unknown name; the options optional
• ctxDrawSymbol: opaque ink in the fill colour, inside its rect and centred in it; over a white background, the rest of the rect left white; false and no ink for an unknown name; the colour following the fill colour; the global alpha applied once; the clip; more ink at a heavier weight; less of a variable symbol at a lower value; a TypeError for a name that is not a string
• the same pixels from a worker as on the main thread

Mutation check, each change rebuilt and run alone, all 7 caught:

mutant	caught by
no SourceIn fill, so the template's black	the fill colour checks
not centred in the rect	the centring check
the weight ignored	the size and ink weight checks
variableValue ignored	the variable symbol check
no transparency layer, the fill flooding the rect	the background check
no TypeError for a non-string name	the TypeError check
true for an unknown name	the unknown name check

CI also gains a step for test/text-features.js, which #70 added to the test script but not to the workflow.

npm test on this Mac: every test file passes.