stream: reduce per-chunk work in WHATWG tee and BYOB reads

[mcollina]

Two per-chunk reductions in the WHATWG streams implementation, continuing the series from #64431 / #64451.

1. Ring buffer for pending BYOB pull-into descriptors

The byte controller's [[pendingPullIntos]] list was the last per-chunk queue still backed by a plain array consumed with ArrayPrototypeShift. BYOB reads push and shift one descriptor per read, and Array.prototype.shift has real per-call cost even at length 1. This backs the list with the same lazily materialized Queue ring buffer used for the request queues in #64431, so constructing a byte stream still allocates no descriptor storage.

2. Reused tee read request

Both ReadableStreamDefaultTee and the byte tee's default-reader pull path allocated a fresh read request object (three computed-symbol methods) plus a fresh forwarding microtask closure for every chunk. Only one read is ever in flight per tee (guarded by the reading flag), so a single read request and forwarding function are now shared across all chunks, with the chunk handed over through a captured slot. The request is materialized lazily on the first pull so tee() itself stays allocation-light.

A steady-state tee benchmark is added (benchmark/webstreams/tee.js); tee previously only had creation coverage.

Benchmarks (benchmark/compare.js, 12–20 runs each, quiet machine):

                                                            confidence improvement accuracy
webstreams/tee.js type='normal' n=100000                           ***    +79.78 %  ±11.02%
webstreams/tee.js type='bytes' n=100000                            ***    +29.38 %   ±5.05%
webstreams/readable-read.js type='byob' n=100000                    **     +2.45 %   ±3.06%

All other rows (readable-read normal, read-buffered, async-iterator, creation incl. ReadableStream.tee, js_transfer) are neutral; the eager-request version of the tee change initially regressed tee creation by 24 % which is what motivated the lazy materialization (row now +0.49 % n.s.).

Gates: WPT streams/compression/encoding plus the whatwg/webstreams parallel suites all pass. One white-box test that planted a plain array as [[pendingPullIntos]] was updated to build the real Queue representation.

[nodejs-github-bot]

Review requested:

• @nodejs/performance

[codecov]

Codecov Report

❌ Patch coverage is 86.95652% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.15%. Comparing base (41525ab) to head (30a5834).
⚠️ Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/webstreams/readablestream.js	86.95%	14 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64818      +/-   ##
==========================================
- Coverage   90.16%   90.15%   -0.01%     
==========================================
  Files         746      746              
  Lines      242660   242691      +31     
  Branches    45720    45729       +9     
==========================================
+ Hits       218793   218803      +10     
- Misses      15360    15379      +19     
- Partials     8507     8509       +2     

Files with missing lines	Coverage Δ
lib/internal/webstreams/readablestream.js	98.20% <86.95%> (+0.01%)	⬆️

... and 33 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/75293/