perf(postcss-discard-duplicates): bucket rules by selector

[dkryaklin]

What

Replaces the O(N²) duplicate-rule scan in postcss-discard-duplicates with a single selector-bucketing pass: each rule dedupes only against earlier rules sharing its selector instead of scanning every preceding sibling.

Full default preset over a corpus of 21 released framework CSS files (master = 100% of current time, lower is faster):

	master	this PR
total time	100%	93% (−7%)

The scan is quadratic even when there are no duplicates, so the win grows with rule count. Synthetic stylesheets of N distinct-selector rules (this plugin only):

rules (N)	master	this PR
500	100% (1.69 ms)	71% (1.20 ms)
1000	100% (4.84 ms)	52% (2.52 ms)
2000	100% (13.78 ms)	43% (5.90 ms)
4000	100% (45.16 ms)	28% (12.78 ms)

Document order and last-wins semantics are unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.80%. Comparing base (a282215) to head (7d8bcc3).
⚠️ Report is 49 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1815      +/-   ##
==========================================
- Coverage   98.82%   98.80%   -0.02%     
==========================================
  Files         124      126       +2     
  Lines       10776    11305     +529     
  Branches       15       15              
==========================================
+ Hits        10649    11170     +521     
- Misses        127      135       +8     

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