This PR is stacked on #65143 — please review the last two commits only.

Two independent optimizations for WHATWG streams:

TextEncoderStream: encode whole chunks natively. The encode-and-enqueue transform walked every code unit in JS, materializing a one-character string per index and building the output via string concatenation. The only state that crosses chunks is a trailing high (leading) surrogate, and TextEncoder.encode()'s USVString conversion already replaces interior lone surrogates with U+FFFD exactly like the spec loop. The transform now joins a pending high surrogate, holds back a new trailing one, and encodes the rest in a single native call. Verified byte-identical to the previous algorithm over 200k randomized surrogate-heavy chunk sequences, plus the full WPT encoding suite. The streaming decode path also stops allocating a { stream: true } options object per chunk.

ReadableStream.from(): drop the async pull machinery. The pull algorithm was an async function awaiting iterator.next() and then the value — an async frame, two await wrappers, and a controller-side reaction per chunk. It is now callback-style with per-stream cached reaction steps, delivering completion directly to the controller's cached pull reactions (the parked-algorithm-result contract from #65143). Thenable adoption is preserved, including the observable .then lookup on plain object values. The iterator's next method is now looked up once at setup, matching the spec's GetIteratorDirect.

Benchmark results (benchmark/compare.js --runs 10, new rows added since the suite covered neither path):

                                                                confidence improvement accuracy (*)    (**)   (***)
webstreams/encoding-streams.js len=1024 kind='encode' n=100000        ***    546.04 %      ±16.30% ±23.40% ±34.38%
webstreams/encoding-streams.js len=16 kind='encode' n=100000          ***     19.93 %       ±4.31%  ±5.96%  ±8.26%
webstreams/from.js kind='sync' n=1000000                              ***     27.39 %       ±2.15%  ±2.96%  ±4.06%
webstreams/from.js kind='async' n=1000000                             ***     23.30 %       ±2.74%  ±3.76%  ±5.13%

All other webstreams rows (pipe-to ×9, pipe-through, read/read-buffered, async-iterator, tee, creation, js_transfer, decode) are unchanged. Full WPT streams + encoding suites and the parallel webstreams/whatwg test batches pass.