feat: add max-average-specificity rule

[bartveneman]

Closes #77

Summary

• New rule projectwallace/max-average-specificity that tracks the specificity of all selectors and reports when the average exceeds the configured [a, b, c] maximum
• Primary option is a tuple of 3 non-negative numbers (floats allowed), e.g. [0, 2.5, 1]
• Uses calculateSpecificity and compareSpecificity from @projectwallace/css-analyzer
• Uses ruleFunction.primaryOptionArray = true so stylelint correctly treats the tuple as the primary option (not [primaryOption, secondaryOptions])
• Added to recommended config with value [0, 2.5, 1]
• Registered in src/index.ts and documented in README.md per CONTRIBUTING.md

Test plan

• Invalid options (not an array, wrong length, negative values) produce invalidOptionWarnings
• No selectors → no violation
• Average specificity within limit → no violation
• Average specificity equal to limit → no violation
• Average specificity exceeding limit → violation reported
• Float values in primary option work correctly
• All 310 tests pass

https://claude.ai/code/session_015qh5mN6GXk2cUWQy2JFV7L

[codecov-commenter]

Bundle Report

Changes will increase total bundle size by 2.05kB (3.54%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
stylelintPlugin-esm	59.96kB	2.05kB (3.54%) ⬆️

Affected Assets, Files, and Routes:

view changes for bundle: stylelintPlugin-esm

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
index.mjs	1.93kB	55.9kB	3.57%
configs/recommended.d.mts	56 bytes	1.55kB	3.76%
configs/recommended.mjs	70 bytes	1.47kB	5.01% ⚠️

Files in index.mjs:

• ./src/index.ts → Total Size: 940 bytes

• ./src/rules/max-average-specificity/index.ts → Total Size: 1.75kB

Files in configs/recommended.mjs:

• ./src/configs/recommended.ts → Total Size: 1.42kB

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.17%. Comparing base (e91b663) to head (5453869).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #86      +/-   ##
==========================================
+ Coverage   98.11%   98.17%   +0.06%     
==========================================
  Files          30       31       +1     
  Lines         847      878      +31     
  Branches      232      238       +6     
==========================================
+ Hits          831      862      +31     
  Misses         13       13              
  Partials        3        3              

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