perf: remove redundant seen-Set deduplication in merge_entry_ranges

[bartveneman]

merge_ranges already eliminates exact duplicates via sort-and-merge, so the seen Set and its string-key allocations were unnecessary.

[codecov-commenter]

Bundle Report

Changes will decrease total bundle size by 163 bytes (-0.63%) ⬇️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
index.js-esm	11.8kB	-163 bytes (-1.36%) ⬇️

Affected Assets, Files, and Routes:

view changes for bundle: index.js-esm

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
index.js	-163 bytes	10.45kB	-1.54%

Files in index.js:

• ./src/lib/decuplicate.ts → Total Size: 937 bytes

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.52%. Comparing base (12bd9d8) to head (30df4f0).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
- Coverage   96.54%   96.52%   -0.03%     
==========================================
  Files          14       14              
  Lines        1014     1008       -6     
  Branches      135      133       -2     
==========================================
- Hits          979      973       -6     
  Misses         34       34              
  Partials        1        1              

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