feat(live-preview): add grip-handle drag reorder for list items

[Harriet-star-glitch]

Summary / 摘要

Adds a small grip handle before each list item's bullet/number in live preview; dragging it reorders the item among its siblings (nested sub-lists move with their parent, preserving indentation).

Motivation / 背景与动机

• Issue:
• Roadmap (docs/ROADMAP.md): not tracked as its own top-level row; tracked in the OpenSpec change below (task 2.4, P1 Advanced Editing)
• OpenSpec change: add-missing-features (task 2.4 marked done in this PR)

Changes / 变更内容

• packages/core:
  • live-preview.ts: grip handle + custom mousedown/mousemove/mouseup drag state machine inside buildListDecorations (no HTML5 DnD, per CLAUDE.md rule 5). New pure, unit-testable getListItemRanges() / computeListReorderChange() helpers so the reorder semantics are covered without needing to simulate pixel-accurate drag coordinates in jsdom.
  • live-preview-table.ts: generalized the table widget's editing-lock counter (previously table-only, isTableEditing) into isStructuralEditing() / acquireStructuralEditingLock() / releaseStructuralEditingLock(), since list drag needs the same "skip live-preview StateField rebuilds mid-interaction" guarantee the table widget already relied on (CLAUDE.md rules 8/12). Internal only — not exported from the package's public index.ts.
• openspec/: add-missing-features/tasks.md — checked off 2.4.

Testing / 测试

• pnpm test passes (527/527, including 12 new tests)
• Affected packages build (pnpm build)
• New / updated vitest cases: packages/core/test/live-preview-list-drag.test.ts — grip rendering (top-level list, nested sub-list, and the cursor-revealed-raw-markup case), the pure reorder computation (move down / move up / no-op / out-of-range / preserves nested indentation), and a real EditorView round-trip applying the computed change.
• Manual UI check in electron-demo: built the app and drove the packaged Electron binary with a throwaway Playwright script (not committed) — typed a real list via keyboard, dragged a grip with real mouse events, confirmed the drop indicator rendered mid-drag and the document text reordered correctly (alpha,beta,gamma → beta,gamma,alpha for a drag of item 0 past item 2), and confirmed no console/page errors.

Compliance / 合规自检

• CLA signed — will sign if/when the bot prompts
• AI disclosure: this PR's functional code is primarily AI-generated. I asked Claude (Anthropic's Claude Code) to survey this repo's open roadmap items, pick an unclaimed one, and design + implement it end-to-end, including the shared editing-lock refactor and the new test file. I chose the task, reviewed the diff, test output, and the manual verification recording, but have not independently rewritten the implementation. I'm disclosing this explicitly per GOVERNANCE.md §6.2 rather than checking "not AI-generated" — happy to walk through and defend any part of it in review, and understand maintainers may decide this doesn't meet the bar in §6.2.
• New dependencies: none
• No build artifacts committed
• No secrets committed

Checklist / 自检清单

• Title follows Conventional Commits
• Public API changes update package README — n/a, no public API surface changed (isStructuralEditing etc. stay internal to packages/core/src, not exported from index.ts)
• Touched live-preview-table.ts → only renamed/generalized the shared lock counter; did not touch any of the 12 Table Widget drag/DOM rules in CLAUDE.md
• New capability / breaking change → this completes an existing, already-approved OpenSpec item (add-missing-features feat(electron): i18n for renderer UI driven by language setting #2.4) rather than opening a new proposal
• Change aligns with project scope

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