[bartlomieju]

Summary

A JS transformer's transform() was invoked through WebIDL's Promise<undefined>
return converter and its result chained with transformPromiseWith(), so every
chunk through a non-identity TransformStream allocated two promises and took
a microtask hop — even when transform() completed synchronously. Only the
identity transform had a fast path.

This invokes transform() directly and returns the synchronous-completion
sentinel (undefined) on any non-thenable result, so
transformStreamDefaultControllerPerformTransform resolves the write with a
shared resolved promise and no microtask hop — mirroring the existing identity
fast path and the default controller's synchronous-pull fast path (#35843). A
synchronous throw becomes a rejected promise (the stream errors as before); a
thenable return keeps the full async path (matching
webidl.invokeCallbackFunction's returnsPromise behavior).

TextEncoderStream and TextDecoderStream transforms are made synchronous
(return undefined instead of PromiseResolve()) so they take this path.

Benchmarks

macOS arm64, best-of-3, 100k chunks. Baseline is this branch with the change
reverted (identical build config).

workload	baseline	this PR	Δ
generic non-identity transform pipe	2.39M/s	2.54M/s	+6%
3× transform chain	1.12M/s	1.24M/s	+11%
TextEncoderStream → TextDecoderStream pipe	1.06M/s	1.13M/s	+6.6%

For reference (Bun 1.4 canary): generic transform 2.14M/s, 3× chain 1.03M/s,
Text En/Decode 1.21M/s. This change moves generic transform pipes past Bun;
the remaining Text En/Decode gap is core.encode/TextDecoder.decode op cost
(native in Bun), not the stream machinery.

Correctness / notes

• Same class of timing change as the identity and sync-pull fast paths (write
  resolves synchronously instead of a microtask later). Reuses the existing
  resolvedPromise() used by the identity path.
• TE/TD correctness verified: encode/decode round-trip, surrogate pair split
  across chunks, lone high surrogate → replacement char, streaming multi-byte
  UTF-8 split across byte chunks, fatal decode error propagation, input
  coercion.
• tests/unit/text_encoding_test.ts + tests/unit/streams_test.ts: 89/89 pass.
• Relying on CI for full WPT streams + encoding coverage.