refactor(core): standardize Widget API with EditorWidget base class

[bunbun79]

Summary

Unify the two separate widget systems under a shared EditorWidget base class. Replace the bare tableEditingCount global with isWidgetEditing(). Add lifecycle hooks and WidgetContext.

Motivation

• Roadmap: P1 "Widget API 标准化" (docs/ROADMAP.md feat(search): support regex queries #9)
• OpenSpec: openspec/changes/add-widget-api-standardization/

Before this PR, Nexus had two independent widget systems with zero shared infrastructure. The bare tableEditingCount global was untestable. Plugin widgets had no lifecycle hooks or lock management.

Changes

File	Change
packages/core/src/widget-base.ts	New — EditorWidget abstract base class
packages/core/src/widget-state.ts	New — isWidgetEditing() / lock management
packages/core/src/widget-context.ts	New — WidgetContext interface
packages/core/src/live-preview-table.ts	EditableTableWidget extends EditorWidget
packages/core/src/widget-extension.ts	NexusWidget extends EditorWidget + lifecycle
packages/core/src/types.ts	Added WidgetLifecycleHooks, deprecated destroy
packages/core/test/live-preview.test.ts	6 new table clipboard/selection tests

Architecture

Before: WidgetType (bare CM6)       After: EditorWidget (locks + lifecycle)
        ├─ EditableTableWidget             ├─ EditableTableWidget
        └─ NexusWidget                     └─ NexusWidget
        tableEditingCount (global)         isWidgetEditing()

Testing

• pnpm test — 521 tests, 34 files, 0 failures
• TypeScript — zero errors
• OpenSpec proposal at openspec/changes/add-widget-api-standardization/

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