next/font/local custom font-family declaration causes mismatch between @font-face and className CSS

Link to the code that reproduces this issue

https://github.com/oskari/next-local-font-family-mismatch

To Reproduce

1. Clone the repo
2. Run npm install && npm run dev
3. Open browser DevTools → Elements → Styles
4. Inspect the <h1> element
5. Note: @font-face has font-family: "My Custom Font"
   but the class has font-family: myCustomFont (mismatch!)

Current vs. Expected behavior

Current (broken):

The generated CSS has mismatched font-family names:

/* @font-face uses the custom declaration value (correct) */
@font-face {
  font-family: My Custom Font;
  src: url(...) format("woff2");
}

/* But .className uses the JS variable name (wrong!) */
.mycustomfont_abc123__className {
  font-family: myCustomFont, sans-serif;  /* Note: no space, uses variable name! */
}

The font doesn't apply because My Custom Font ≠ myCustomFont.

Expected:

Both should use the same font-family name from the custom declaration:

@font-face {
  font-family: 'My Custom Font';
  src: url(...) format("woff2");
}

.mycustomfont_abc123__className {
  font-family: 'My Custom Font', sans-serif;
}

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.2.0: Tue Nov 18 21:09:40 PST 2025; root:xnu-12377.61.12~1/RELEASE_ARM64_T6000
  Available memory (MB): 32768
  Available CPU cores: 10
Binaries:
  Node: 22.21.0
  npm: 10.9.4
  Yarn: 1.22.22
  pnpm: 10.8.0
Relevant Packages:
  next: 16.2.0-canary.1 // Latest available version is detected (16.2.0-canary.1).
  eslint-config-next: N/A
  react: 19.2.3
  react-dom: 19.2.3
  typescript: 5.9.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Font (next/font)

Which stage(s) are affected? (Select all that apply)

next dev (local), next build (local)

Additional context

Root Cause (Turbopack):

In crates/next-core/src/next_font/local/:

1. stylesheet.rs correctly uses the custom font-family declaration for @font-face rules
2. But util.rs (build_font_family_string) always uses options.font_family() which returns the JS variable name, ignoring custom declarations
3. Similarly, font_fallback.rs uses the variable name for fallback font naming

Start with crates/next-core/src/next_font/local/util.rs and inspect build_font_family_string, then compare it with stylesheet.rs and font_fallback.rs. Run the linked reproduction with npm install && npm run dev and inspect the generated CSS; done means the custom font-family declaration is used consistently by both @font-face and the className CSS.