SwiftMathExample: font switcher + LaTeX playground, fix wide-formula clipping

[kostub]

Summary

Brings the Swift example app closer to the ObjC iosMathExample and fixes a layout bug surfaced while doing so.

New: Playground tab

• A live LaTeX text editor — type any expression and see it render immediately.
• A font switcher (Latin Modern / TeX Gyre Termes / XITS) via a segmented picker.
• The selected font is shared state in ContentView, so switching it in the Playground also re-renders the Examples and Gallery tabs (matching the global font switch in the ObjC example).

Fix: left-clipping of formulas and titles in the Examples tab

Previously, formulas and their plain-text titles were clipped on the left — but only with Latin Modern.

Root cause: MTMathUILabel won't size narrower than its intrinsic (natural formula) width. The Rogers–Ramanujan continued fraction renders ~405pt wide in Latin Modern — wider than the screen — so that single card stretched the whole VStack column past the viewport, and the vertical ScrollView centered the over-wide column, pushing every card's left edge (titles included) off-screen. In TeX Gyre the same formula is narrower than the screen, so nothing overflowed.

Fix:

• Both platform representables now implement sizeThatFits to fill the offered width without demanding more (so a label never forces its container wider than the viewport), falling back to the natural width only when offered unbounded space. This also fixes the same latent overflow in the Gallery tab.
• Each Examples card wraps its formula in a horizontal ScrollView, so a genuinely over-wide formula scrolls instead of breaking the layout.
• .center/.right alignment in the Gallery is preserved (verified on-device).

Testing

• Verified on the iOS Simulator (iPhone 16): Examples and Gallery tabs render with no clipping; titles fully visible; wide formulas contained/scrollable; Gallery center/right alignment intact.
• SwiftMathExample (iOS) and SwiftMathExampleMac (macOS) schemes both build cleanly.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request restructures the SwiftMathExample app by adding a new "Playground" tab for live LaTeX rendering and font selection, sharing the selected font across all tabs. It also introduces horizontal scrolling for wide formulas and implements a custom width policy (sizeThatFits) for math labels on iOS and macOS. The review feedback highlights a critical performance issue in MathFont.font(size:) where instantiating MTFontManager() directly bypasses the internal font cache, causing disk reads on every render; using the shared singleton MTFontManager.fontManager() is recommended to prevent frame drops.