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

[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 (c24e552) to head (f843e46).
⚠️ Report is 5 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 90.05% 90.05% =======================================
Files 714 714 Lines 225247 225338 +91 Branches 42578 42597 +19 =======================================
+ Hits 202842 202930 +88 - Misses 14181 14190 +9 + Partials 8224 8218 -6 

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 33 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/73439/

[nodejs-github-bot]

Landed in 2edd842