test(font-subsetting): e2e atomic-loader + subsetting for headless fonts

[Hotell]

Summary

End-to-end integration proving the full headless font pipeline works when the atomic loader and the font-subsetting plugin are combined.

What it verifies

A new webpack test entry (e2eBarrelHeadlessFonts) in the font-subsetting plugin:

1. Starts from a plain barrel import — import { … } from '@fluentui/react-icons'.
2. The atomic loader ({ headless: true, iconVariant: 'fonts' }) rewrites it into headless font atoms (@fluentui/react-icons/headless/fonts/*).
3. The font-subsetting plugin recognizes those headless atoms and subsets the shared font → emitted fonts stay under the 1.5 KiB threshold (from MB-scale originals).
4. Asserts the module graph stays Griffel-free (no @griffel/* module in the graph).

The existing headlessAtoms entry now also carries the Griffel-free assertion, giving both headless-fonts entries the same guarantee.

Changes

• test/src/e2e-barrel-headless-fonts.js — the barrel fixture (+ the required headless/fonts/styles.css import).
• test/webpack.config.js — new e2eBarrelHeadlessFonts entry (scoped atomic-loader rule referencing the loader's built lib via relative path) and a shared assertNoGriffel path. Entries that assert Griffel-freedom disable module concatenation (concatenateModules: false) so the verify hook can inspect individual module resources — otherwise scope hoisting would merge modules into a ConcatenatedModule and hide a leaked @griffel module.
• project.json — the plugin's test target now dependsOn the atomic loader's build. Because the test references the loader through a raw filesystem path (not a package import), Nx's project-graph inference can't detect the dependency; without this, CI (nx affected -t test) wouldn't build the loader's lib/ when only this package is affected, causing a Module not found failure.
• README.md — tip on pairing the subsetting plugin with the atomic loader so consumers don't hand-write headless atomic imports.

Notes

• No package devDependencies added (monorepo single-version policy); the loader is referenced via its built lib path, mirroring how the loader's own tests reference ../lib.
• lib/ is gitignored (source/test-only commit).

[github-actions]

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