[Typography] Add applyRobotoLetterSpacing option to fix letter spacing with CSS variable fontFamily

[starboyvarun]

Fixes #47575

Problem

The Next.js font optimization guide recommends setting typography.fontFamily: 'var(--font-roboto)' in the theme. However, createTypography applies Roboto-specific letter spacing only when fontFamily === defaultFontFamily (strict string equality). A CSS variable reference never matches that string, so the letter spacing is silently dropped — even though the font in use is still Roboto.

This was identified by @Janpot in #47575. A previous PR (#47627) attempted heuristic string detection which was rightly rejected as too fragile.

Solution

Add an explicit applyRobotoLetterSpacing option to typography (option 2 from @Janpot's comment). It defaults to fontFamily === defaultFontFamily, preserving existing behaviour for all current users. Callers who reference Roboto via a CSS variable can opt in:

createTheme({
  typography: {
    fontFamily: 'var(--font-roboto)',
    applyRobotoLetterSpacing: true,
  },
})

• No heuristics, no string matching
• Fully backwards compatible — existing themes are unaffected
• Explicit, documentable, and easy to understand

Changes

• createTypography.js — extract applyRobotoLetterSpacing (default fontFamily === defaultFontFamily) and use it in buildVariant
• createTypography.d.ts — add type with JSDoc
• nextjs.md — add applyRobotoLetterSpacing: true to both App Router and Pages Router font optimization examples

[code-infra-dashboard]

Deploy preview

• docs/data/material/integrations/nextjs/nextjs.md

Bundle size

Bundle	Parsed size	Gzip size
@mui/material	🔺+29B(+0.01%)	🔺+16B(+0.01%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/private-theming	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

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[mj12albert]

IMO this is too niche to justify adding to the API surface #47575 (comment)

[siriwatknp]

I'm closing this PR. Would you mind creating a separate PR to update the docs instead?

Thank you for your effort.