fix: migrate to @eslint/css-tree to support CSS variables (var())

[srobinson]

• Replace css-tree with @eslint/css-tree v3.6.6
• Add var() detection to skip validation for CSS custom properties
• Update all imports across 8 files
• Fix TypeScript types for walk() callbacks
• Remove @types/css-tree dependency

Resolves issue where properties containing var() would report validation
errors even though expansion was successful. CSS variables are runtime
values that cannot be validated at parse time.

[Copilot]

Pull Request Overview

This PR migrates from the original css-tree library to @eslint/css-tree v3.6.6 to add support for CSS custom properties using var(). The migration addresses a validation issue where properties containing var() would incorrectly report errors even though expansion was successful, since CSS variables are runtime values that cannot be validated at parse time.

Key changes:

• Migrated from css-tree peer dependency to @eslint/css-tree v3.6.6 as a regular dependency
• Added containsVar() helper function to detect and skip validation for declarations with CSS variables
• Fixed TypeScript types for walk() callbacks by changing parameter type from csstree.Value to csstree.CssNode with type guards

Reviewed Changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
package.json	Changed from peerDependency css-tree to regular dependency @eslint/css-tree, removed @types/css-tree
pnpm-lock.yaml	Updated package resolutions and dependencies, mdn-data bumped from 2.12.2 to 2.23.0
src/core/validate.ts	Added containsVar() function and logic to skip validation for declarations containing CSS variables
src/internal/layer-parser-utils.ts	Updated import and fixed walk callback types
src/internal/is-value-node.ts	Updated type import
src/handlers/transition/transition-layers.ts	Updated import and fixed walk callback types
src/handlers/mask/mask-layers.ts	Updated import and fixed walk callback types
src/handlers/grid/expand.ts	Updated import and fixed walk callback types
src/handlers/background/background-layers.ts	Updated import and fixed walk callback types
src/handlers/animation/animation-layers.ts	Updated import and fixed walk callback types
docs.llm/llm_src.txt	Documentation file reflecting source code changes

Files not reviewed (1)

• pnpm-lock.yaml: Language not supported

Comments suppressed due to low confidence (1)

src/core/validate.ts:5

• The documentation comment still references 'css-tree library' but should be updated to '@eslint/css-tree' to reflect the migration.

 * This function parses CSS using the css-tree library and validates each CSS property

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[Copilot]

The documentation comment references 'css-tree' but should be updated to '@eslint/css-tree' for consistency with the migration.

Suggested change

	* CSS variables cannot be validated by css-tree as they are runtime values.
	* CSS variables cannot be validated by @eslint/css-tree as they are runtime values.