Allow transform to return a stream

[cpruijsen]

Summary

Add Minipass to the transform return type: ReadEntry | Minipass | null | undefined | false | 0 | '' (same union style as #459). Runtime unpack already pipes a truthy transform result (entry.pipe(tx); tx.pipe(stream)).

transform is documented as returning a stream or a falsey value. v7.5.12 narrowed the TypeScript return type from any to ReadEntry. #459 restored falsey returns (null | undefined | false | 0 | ''), but returning a MiniPass stream still type-errors.

MiniPass is what the README tells callers to return; unpack.ts on('data', (chunk: Buffer) => …) stays well-typed. Alternatives: revert the return type to any, or accept Node duplex streams (NodeJS.ReadWriteStream / stream.Duplex). #459 expanded the union to the documented values instead of restoring any. Happy to switch to a wider Node stream type or any if that is preferred. Node core Transform still type-errors after this change.

Fixes #462

Test plan

• Issue reproduction (extract({ transform: () => new Minipass({ async: true }) })) type-errors on v7.5.22
• Same assignment compiles after the union change
• Type test fails without the source change and passes with it (test/transform-type-check.ts)
• Returning the entry / undefined / false still typechecks (transform type error when falsy value returned #459)
• npm test -- -c -t0: 21247/21247 assertions pass, including test/transform-type-check.ts and test/unpack.js