Make WritableStreamDefaultWriter.write contravariant in its chunk type

[Copilot]

WritableStreamDefaultWriter<W> declared write as a method, so its parameter was compared bivariantly. W reaches WritableStream<W> only through getWriter(), so the entire stream type was bivariant in its chunk type, accepting unsound assignments:

const stream: WritableStream<{ a: string }> = new WritableStream<{ a: string; b: string }>();
stream.getWriter().write({ a: "string" }); // no error, but the sink requires `b`

new ReadableStream<{ a: string }>().pipeTo(new WritableStream<{ a: string; b: string }>()); // no error

W is only ever consumed, never produced, so bivariance buys nothing here.

Changes

• lib.dom.d.ts / lib.webworker.d.ts: declare write as a function property (write: (chunk?: W) => Promise<void>;), making WritableStream<W> contravariant in W under strictFunctionTypes. Both cases above now error; widening in the sound direction (WritableStream<{ a }> where WritableStream<{ a; b }> is expected) still compiles.
• New compiler test writableStreamChunkVariance.ts plus baselines, committed pre-fix (no errors) and then updated, so the behavior change is visible in the diff.

These lib files are generated from TypeScript-DOM-lib-generator; the same change needs to be mirrored upstream to survive regeneration.

• Fixes Consider making WritableStreamDefaultWriter.write() contravariant #59012