stream: add fast paths for webstreams read and pipeTo

[mcollina]

Add internal fast paths to improve webstreams performance without changing the public API or breaking spec compliance.

1. ReadableStreamDefaultReader.read() fast path: When data is already buffered in the controller's queue, return PromiseResolve() directly without creating a DefaultReadRequest object. This is spec-compliant because read() returns a Promise, and resolved promises still run callbacks in the microtask queue.

2. pipeTo() batch read fast path: When data is buffered, batch reads directly from the controller queue up to highWaterMark without creating PipeToReadableStreamReadRequest objects per chunk. Respects backpressure by checking desiredSize after each write.

Benchmark results:

• pipeTo: ~11% faster (***)
• buffered read(): ~17-20% faster (***)

This was done in partnership with Vercel to improve the performance of React and Next.js, and from a conversation on X with @cramforce.

[nodejs-github-bot]

Review requested:

• @nodejs/performance
• @nodejs/web-standards

[codecov]

Codecov Report

❌ Patch coverage is 98.71795% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.71%. Comparing base (ae2ffce) to head (76ccbaf).
⚠️ Report is 92 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/webstreams/readablestream.js	98.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61807      +/-   ##
==========================================
- Coverage   89.75%   89.71%   -0.05%     
==========================================
  Files         674      675       +1     
  Lines      204416   204884     +468     
  Branches    39285    39377      +92     
==========================================
+ Hits       183472   183807     +335     
- Misses      13227    13338     +111     
- Partials     7717     7739      +22     

Files with missing lines	Coverage Δ
lib/internal/webstreams/readablestream.js	98.45% <98.71%> (+<0.01%)	⬆️

... and 70 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71335/

[mcollina]

@jasnell @Qard Can I get another approval?

[MattiasBuelens]

Comparing batchCount to hwm is only valid if controller[kState].sizeAlgorithm is the default size algorithm (i.e. each chunk has size 1). I don't think we can assume that?

We should add a test that uses a custom size algorithm.

[MattiasBuelens]

Why || 1? An HWM of 0 is valid.

[gurgunday]

lgtm

[mertcanaltin]

LGTM