Headless agent edits corrupt the document: cloneDocument JSON-clones away Maps/ArrayBuffers, breaking export + dropping images

[aldrinjenson]

Describe the bug

cloneDocument runs on every headless agent edit (executeCommand → cloneDocument) and deep-clones via JSON.parse(JSON.stringify(doc)). JSON can't represent several fields the document model carries, so the clone silently corrupts them:

• package.headers / package.footers / package.media are Maps → become {}
• originalBuffer and each MediaFile.data are ArrayBuffers → become {}
• package.properties.created / modified are Dates → become strings

The result is load-bearing: a no-edit export works, but after the first edit export throws and images are lost.

To Reproduce

import { DocumentAgent } from '@eigenpal/docx-editor-core/headless';

const agent = await DocumentAgent.fromBuffer(bytes);   // any real .docx
await agent.toBuffer();                                  // ✅ works (no edit)

const edited = agent.insertText({ paragraphIndex: 0, offset: 0 }, 'Hello ');
await edited.toBuffer();                                 // ❌ throws

Depending on the document, the throw is one of:

• Can't read the data of 'the loaded zip file' — repackDocx → JSZip.loadAsync(originalBuffer) on the {} left where the ArrayBuffer was.
• map.entries is not a function — repackDocx's collectParts iterating package.headers / footers, now {} instead of Maps.

Any document with images also loses every image on the first edit (the media Map is gone). insertTable(...).toBuffer() fails the same way.

Expected behavior

A headless generate → edit → export round-trip should produce a valid .docx (and keep headers/footers and images), since this is the documented fromBuffer → edit → toBuffer flow.

Root cause

cloneDocument (packages/core/src/agent/executor/helpers.ts) uses JSON.parse(JSON.stringify(doc)), which cannot round-trip Map, ArrayBuffer, or Date.

Additional context

• Affects the fully headless / server-side workflow (generate + edit + export without ever opening the editor).
• Reproduced on @eigenpal/docx-editor-core 1.9.0.
• Fix proposed in fix(core): preserve Maps and buffers when cloning a document for edits #990 (clone with structuredClone; share the read-only originalBuffer and shallow-copy the immutable media map so large binaries aren't recopied per edit).

[aldrinjenson]

Fix proposed in #990 — clones with structuredClone and adds a cloneDocument unit test plus a fromBuffer → edit → toBuffer round-trip regression test.