stream: cache minimum cursor count in share

[trivikr]

Description

This updates share() and shareSync() to avoid recomputing the minimum
consumer cursor on every buffer trim attempt.

Instead of scanning all consumers each time, share now caches the current
minimum cursor and tracks how many consumers are positioned at that
cursor. The minimum is recomputed only when the last consumer at the
cached minimum advances or detaches.

The shared getMinCursor() utility now returns both the minimum cursor
and the number of consumers at that cursor.

Benchmark

                                                                                     confidence improvement accuracy (*)   (**)  (***)
streams/iter-throughput-share.js n=5 backpressure='block' batches=10000 consumers=2         ***      6.06 %       ±2.01% ±2.76% ±3.76%
streams/iter-throughput-share.js n=5 backpressure='block' batches=10000 consumers=32        ***     69.76 %       ±1.56% ±2.15% ±2.96%
streams/iter-throughput-share.js n=5 backpressure='block' batches=10000 consumers=8         ***     20.96 %       ±1.71% ±2.38% ±3.35%

---

Assisted-by: openai:gpt-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/performance
• @nodejs/streams

[trivikr]

I'd attempted making the same changes in broadcast. They were reverted since benchmarks showed regression

                                                                                       confidence improvement accuracy (*)    (**)   (***)
streams/iter-throughput-broadcast.js n=5 datasize=1048576 consumers=1 api='classic'                   -4.72 %       ±8.35% ±11.66% ±16.39%
streams/iter-throughput-broadcast.js n=5 datasize=1048576 consumers=1 api='iter'              ***    -17.21 %       ±8.78% ±12.06% ±16.50%
streams/iter-throughput-broadcast.js n=5 datasize=1048576 consumers=1 api='webstream'         ***    -31.77 %      ±12.75% ±17.99% ±25.68%
streams/iter-throughput-broadcast.js n=5 datasize=1048576 consumers=2 api='classic'                   -2.32 %       ±5.56%  ±7.76% ±10.90%
streams/iter-throughput-broadcast.js n=5 datasize=1048576 consumers=2 api='iter'               **    -11.18 %       ±6.34%  ±8.72% ±11.95%
streams/iter-throughput-broadcast.js n=5 datasize=1048576 consumers=2 api='webstream'         ***    -22.39 %       ±6.13%  ±8.68% ±12.44%
streams/iter-throughput-broadcast.js n=5 datasize=1048576 consumers=4 api='classic'                   -5.51 %       ±7.07%  ±9.78% ±13.55%
streams/iter-throughput-broadcast.js n=5 datasize=1048576 consumers=4 api='iter'                      -1.68 %       ±6.46%  ±8.92% ±12.30%
streams/iter-throughput-broadcast.js n=5 datasize=1048576 consumers=4 api='webstream'         ***    -26.03 %       ±4.97%  ±6.88%  ±9.54%
streams/iter-throughput-broadcast.js n=5 datasize=16777216 consumers=1 api='classic'            *     -4.21 %       ±3.90%  ±5.36%  ±7.33%
streams/iter-throughput-broadcast.js n=5 datasize=16777216 consumers=1 api='iter'              **    -10.27 %       ±7.18% ±10.03% ±14.07%
streams/iter-throughput-broadcast.js n=5 datasize=16777216 consumers=1 api='webstream'         **    -20.80 %      ±11.70% ±16.63% ±24.05%
streams/iter-throughput-broadcast.js n=5 datasize=16777216 consumers=2 api='classic'                  -4.53 %       ±5.56%  ±7.70% ±10.66%
streams/iter-throughput-broadcast.js n=5 datasize=16777216 consumers=2 api='iter'             ***    -11.46 %       ±5.39%  ±7.40% ±10.13%
streams/iter-throughput-broadcast.js n=5 datasize=16777216 consumers=2 api='webstream'         **    -12.59 %       ±9.01% ±12.36% ±16.87%
streams/iter-throughput-broadcast.js n=5 datasize=16777216 consumers=4 api='classic'                  -4.09 %       ±6.01%  ±8.35% ±11.61%
streams/iter-throughput-broadcast.js n=5 datasize=16777216 consumers=4 api='iter'               *     -8.16 %       ±6.54%  ±8.98% ±12.27%
streams/iter-throughput-broadcast.js n=5 datasize=16777216 consumers=4 api='webstream'        ***     -8.95 %       ±4.03%  ±5.55%  ±7.60%

[codecov]

Codecov Report

❌ Patch coverage is 85.15625% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.05%. Comparing base (78bbee3) to head (2eda9ba).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/streams/iter/share.js	83.33%	19 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #63262    +/-   ##
========================================
  Coverage   90.04%   90.05%            
========================================
  Files         713      714     +1     
  Lines      225003   225333   +330     
  Branches    42536    42605    +69     
========================================
+ Hits       202606   202915   +309     
- Misses      14177    14189    +12     
- Partials     8220     8229     +9     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/broadcast.js	84.66% <100.00%> (+0.02%)	⬆️
lib/internal/streams/iter/utils.js	100.00% <100.00%> (ø)
lib/internal/streams/iter/share.js	84.12% <83.33%> (-0.06%)	⬇️

... and 38 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.