sourcemaps: store source map mappings in a typed array

[jackyzha0]

Why

SourceMap kept one six-element array per mapping segment. Bundled applications have millions of segments, and with --enable-source-maps these arrays live for the life of the process. From our prod application, a 63 MB source map with 2.9 million segments retained about 290 MiB of heap.

What changed

• Store the segments in a single Int32Array, six fields per segment, and resolve source URLs and names through per-map tables.
• Segments are counted before parsing so storage is allocated once, and sorting is skipped when the mappings are already in order.
  • The public API and the objects returned by findEntry() and findOrigin() are unchanged.

Disclaimer: this PR was written with the help of Opus 5.5 but I have personally read, reviewed, and validated the changes.

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[codecov]

Codecov Report

❌ Patch coverage is 98.93048% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.29%. Comparing base (e7d8ab5) to head (2cb056f).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/source_map/source_map.js	99.45%	1 Missing ⚠️
lib/internal/test_runner/coverage.js	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66261      +/-   ##
==========================================
- Coverage   90.30%   90.29%   -0.01%     
==========================================
  Files         789      789              
  Lines      272880   273012     +132     
  Branches    52110    52135      +25     
==========================================
+ Hits       246418   246515      +97     
- Misses      16912    16963      +51     
+ Partials     9550     9534      -16     

Files with missing lines	Coverage Δ
lib/internal/source_map/source_map.js	99.61% <99.45%> (+0.12%)	⬆️
lib/internal/test_runner/coverage.js	59.81% <50.00%> (ø)

... and 36 files with indirect coverage changes

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