Add no-invalid-z-index rule to stylelint plugin

[bartveneman]

Summary

This PR adds a new stylelint rule projectwallace/no-invalid-z-index that validates z-index values are valid 32-bit signed integers within the range of -2147483648 to 2147483647.

Key Changes

• New rule implementation (src/rules/no-invalid-z-index/index.ts): Validates that z-index values are integers within the 32-bit signed integer range, with support for checking fallback values in CSS custom properties
• Comprehensive test suite (src/rules/no-invalid-z-index/index.test.ts): 16 test cases covering valid values (positive, negative, zero, auto), boundary values, invalid values (out of range, floats), var() with fallbacks, and rule disabling
• Documentation (src/rules/no-invalid-z-index/README.md): Clear explanation of the rule with examples of valid and invalid z-index values
• Plugin integration: Added the new rule to the main plugin exports and included it in the recommended configuration

Implementation Details

• Uses the @projectwallace/css-parser library to parse declaration values and extract numeric values
• Validates both direct z-index values and fallback values within var() functions
• Properly handles edge cases like z-index: auto and var() without fallbacks (which cannot be validated)
• Reports violations with the specific invalid value in the error message

https://claude.ai/code/session_01MaWwhMcH7Dv966R834zVUE

[codecov-commenter]

Bundle Report

Changes will increase total bundle size by 1.62kB (2.64%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
stylelintPlugin-esm	62.91kB	1.62kB (2.64%) ⬆️

Affected Assets, Files, and Routes:

view changes for bundle: stylelintPlugin-esm

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
index.mjs	1.52kB	58.56kB	2.67%
configs/recommended.d.mts	50 bytes	1.71kB	3.02%
configs/recommended.mjs	45 bytes	1.61kB	2.88%

Files in index.mjs:

• ./src/index.ts → Total Size: 1.04kB

• ./src/rules/no-invalid-z-index/index.ts → Total Size: 1.38kB

Files in configs/recommended.mjs:

• ./src/configs/recommended.ts → Total Size: 1.57kB

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.14%. Comparing base (5519ae7) to head (3ae83ab).

Files with missing lines	Patch %	Lines
src/rules/no-invalid-z-index/index.ts	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #93      +/-   ##
==========================================
- Coverage   98.20%   98.14%   -0.07%     
==========================================
  Files          33       34       +1     
  Lines         893      918      +25     
  Branches      230      234       +4     
==========================================
+ Hits          877      901      +24     
- Misses         13       14       +1     
  Partials        3        3              

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