[mcollina]

Reverts the two commits that moved parked TransformStream writes off the [[backpressureChangePromise]] reaction, which regressed AsyncLocalStorage propagation into transform() / flush() (#66906):

• f81a148 "stream: avoid promise allocation for parked transform writes" (stream: keep webstream stream states in fast-mode objects #65625, v26.10.0): transform() for every chunk after a parked write, and flush(), run in the reader's context.
• 9ec9383 "stream: decouple transform backpressure changes" (stream: decouple transform backpressure changes #65143, v26.8.0): the first parked transform() runs in the reader's context.

Conflicts with later commits are resolved by keeping their state record classes (TransformStreamState now carries backpressureChange again), the kParkedAlgorithmResult handling still used by ReadableStream.from(), and the kResolvedPromise import used by resolvedStartAlgorithm (3c9f878). test-whatwg-transformstream-parked-write-error.js is kept: it covers spec behavior and passes on the reverted code. The pipe-through benchmark is kept too.

Verified locally on this branch:

repro (issue):                   a:writer b:writer c:writer d:writer e:writer flush:writer
repro, --no-async-context-frame: a:writer b:writer c:writer d:writer e:writer flush:writer
reverse (reader in als.run):     a:none b:none c:none flush:none   (both modes)

The regression test from #66907 passes in both modes; whatwg/webstream tests, WPT streams, async-local-storage and async-hooks suites pass.

Alternative: #66915 (draft) backports the fix from #66907 instead, with a promise fallback for --no-async-context-frame. Only one of the two should land.

Refs: #66906
Refs: #66907
Refs: #66913

🤖 Generated with Claude Code

https://claude.ai/code/session_01LGUJ9PwPYkYJiCpUEsr7KJ

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.75%. Comparing base (ca26282) to head (ac998a5).
⚠️ Report is 1275 commits behind head on v26.x-staging.

Additional details and impacted files

@@                Coverage Diff                @@
##           v26.x-staging   #66914      +/-   ##
=================================================
+ Coverage          90.24%   90.75%   +0.51%     
=================================================
  Files                729      775      +46     
  Lines             242760   273391   +30631     
  Branches           46044    52520    +6476     
=================================================
+ Hits              219073   248117   +29044     
- Misses             15133    16144    +1011     
- Partials            8554     9130     +576     

Files with missing lines	Coverage Δ
lib/internal/webstreams/transformstream.js	98.60% <100.00%> (+0.03%)	⬆️

... and 414 files with indirect coverage changes

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