fix(async): don't crash streaming pushes on pooled Node Buffers (#278)

[maximilliangrand]

Fixes #278.

Repro (Node):

const { Zip, AsyncZipDeflate, unzipSync } = require('fflate');
const zd = new AsyncZipDeflate('a.bin', { level: 6 });
new Zip((e,d,f)=>{}).add(zd);
zd.push(Buffer.from('x'.repeat(2000)), true); // DataCloneError: Cannot transfer object of unsupported type

Cause: The async streaming classes transfer d.buffer on every push. A Node Buffer (from Buffer.from/readFileSync) is a view into the shared pool ArrayBuffer that Node marks untransferable, so the transfer throws. #227 (4b7a6cb) already filters untransferable buffers, but only on the worker's first message; streaming pushes call postMessage directly and bypass it.

Fix: Wrap the node worker's postMessage so the transfer list is filtered on every message; untransferable buffers fall back to a structured clone (the same graceful degradation already used for cross-realm buffers). Fresh, transferable Uint8Arrays still transfer as before.

Tests/types/docs: adds an async regression test (test/5-async.ts, previously empty) covering AsyncZipDeflate and AsyncDeflate with a pooled Buffer; tsc clean; no public API or doc change.