Ensure custom variants can be used via @variant in addBase

[RobinMalfait]

This PR fixes an issue where @variant inside addBase is being used with a custom variant.

The issue is that we substitute the @variant calls immediately when we call the addBase function. That means that variants that aren't processed yet will error out.

This is a regression, because this used to work in Tailwind CSS v4.3.0 and started failing in Tailwind CSS v4.3.1.

Test plan

1. Added a regression test

[greptile-apps]

Confidence Score: 5/5

Safe to merge — the change is minimal and targeted, and the global substitution pass already handles @variant for the rest of the AST.

The fix removes a single redundant early call to substituteAtVariant in the addBase handler and relies on the existing global pass in index.ts (line 673) that runs after all custom variants are registered. Error behaviour for genuinely unknown variants is unchanged — substituteAtVariant still throws if the variant cannot be resolved. The regression test directly covers the broken scenario.

No files require special attention.

_{Reviews (2): Last reviewed commit: "update changelog" | Re-trigger Greptile}

[coderabbitai]

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Review failed

The pull request is closed.

ℹ️ Recent review info

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Run ID: 7f91483a-a04c-40db-84c5-b116bbc803bc

📥 Commits

Reviewing files that changed from the base of the PR and between 7091b2b and 191377a.

📒 Files selected for processing (1)

• CHANGELOG.md

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Walkthrough

In buildPluginApi().addBase, the call to substituteAtVariant(baseNodes, designSystem) and the corresponding OR assignment into featuresRef.current have been removed, along with the substituteAtVariant import from ../variants. Feature tracking inside addBase now only applies substituteFunctions. A new test is added to confirm that @variant directives referencing a @custom-variant-defined variant are correctly resolved in base-layer output, and the changelog documents this fix.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main fix: enabling custom variants to work with @variant in addBase, which matches the PR's primary objective.
Description check	✅ Passed	The description clearly explains the regression issue, root cause (immediate substitution preventing proper variant resolution), and mentions the test plan, all directly related to the changeset.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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