fix(core): preserve async asset insertion anchors

[xiaoxiaowen610]

Summary / 摘要

Preserve paste and drop insertion anchors while asset uploads are pending, then insert each successful batch in one document transaction.

在资源异步上传期间保持粘贴和拖放的原始插入位置，并将同批成功结果通过一次文档事务原子插入。

Motivation / 背景与动机

The default asset fallback previously called replaceSelection() after every upload resolved. That used the selection at completion time rather than the position where the paste or drop started: moving the cursor redirected the Markdown, editing the original selection could overwrite newer content, and multi-file uploads created multiple undo steps.

• Issue: none
• Roadmap: N/A — bug fix
• OpenSpec change: N/A — internal bug fix with no public API or new capability

Changes / 变更内容

• packages/core:
  • Track every initiating selection range in a CodeMirror StateField and map it through document transactions.
  • Preserve selection-boundary and multi-cursor semantics; derive drop anchors from pointer coordinates.
  • Upload files serially, isolate individual failures, and commit successful results in one undoable transaction.
  • Discard stale results after setDocument() or editor destruction, and skip the default pipeline in read-only editors.
• Other packages / apps/electron-demo / openspec: N/A

Testing / 测试

• pnpm typecheck
• pnpm test — 34 files, 551 tests
• pnpm build
• pnpm build:electron-demo
• New Vitest coverage for mapped anchors, touched selections and boundaries, multi-cursor paste, pointer-based drop, atomic batches and undo, partial/all failures, out-of-order batches, setDocument(), composition, read-only mode, and destruction.
• Manual UI check in electron-demo: N/A — the demo does not configure onAssetUpload, and this change adds no visible UI.

Compliance / 合规自检

• CLA signed — I will complete the first-contributor bot prompt after this PR opens.
• AI disclosure: the functional code in this PR is not primarily generated by AI.
  • AI tools assisted with design discussion, edge-case review, and regression-test suggestions. The functional implementation was authored and reviewed by the contributor.
• New dependencies: none.
• No build artifacts committed.
• No secrets, .env files, or personal vault data committed.

Checklist / 自检清单

• Title follows Conventional Commits.
• Public API changes update package README / types: N/A — no public API changes.
• Touched live-preview-table.ts: N/A.
• New capability / breaking change requires OpenSpec: N/A.
• Change aligns with the project scope.

Screenshots / Recordings

N/A — no visible UI changes.

[CLAassistant]

All committers have signed the CLA.