Add \: spacing command alias

[Jason-Abbott]

Add support for the remaining fine spacing commands. \, (thin space) already works, but the others are missing.

Currently returns: Invalid command \: (and similar for others)

Commands to Implement

Command	Description	Width
\:	Medium space	4/18 em
\;	Thick space	5/18 em
\!	Negative thin space	-3/18 em

Examples

\int\!\!\!\int f(x,y) dx dy      % tight double integral
x \, y \: z \; w                  % mixed spacing

Use Cases

• Fine typography control
• Integral notation (negative space for tight double integrals)
• Custom spacing in complex expressions

Implementation Notes

• Insert proper MathSpace atoms with appropriate widths
• \, is already implemented (3/18 em) — use as reference

Update (from TeXShop evaluation)

\; and \! already work — they are mapped in MathAtomFactory.supportedLatexSymbols as:

• ";" → MathSpace(space: 5) (thick space, 5/18 em) ✅
• "!" → MathSpace(space: -3) (negative thin space, -3/18 em) ✅

Only \: is actually missing. It should map to MathSpace(space: 4), same as the existing ">" entry.

Fix

Add to supportedLatexSymbols in Sources/MathViews/MathRender/MathAtomFactory.swift (around line 535):

":" : MathSpace(space: 4),

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