feat(tree-view): add selectNode method

[metonym]

Summary

• Add selectNode(id) imperative method to TreeView
• Allows programmatic node selection without changing focus or expansion state
• Includes TypeScript definitions, unit tests, and documentation

Changes

• Implementation: Added selectNode(id) method in TreeView.svelte
• Types: Updated TypeScript definitions in TreeView.svelte.d.ts
• Tests: Added unit test verifying selection without side effects
• Docs: Created example demonstrating the new method

Use Case

Previously, selecting a node programmatically required either:

• User interaction (clicking)
• Using showNode() which also expands ancestors and changes focus

Now selectNode() provides a lighter-weight option for just updating selection state.

Test plan

• Unit tests pass (npm test -- TreeView.test.ts)
• Method correctly updates selectedIds
• Method does not expand nodes
• Method does not trigger select event
• Documentation example created