feat(createNested): add active state management

[johnleider]

Summary

Adds active/highlighted state management to createNested, independent of selected (checked) and opened (expanded) states. This enables tree views, navigation drawers, and nested lists to track which item is currently "active" (e.g., the file open in an editor sidebar).

Three Independent States

State	Controls	Example
Selected (checked)	v-model / checkbox	Files checked for bulk delete
Opened (expanded)	v-model:opened / toggle	Folder showing children
Active (highlighted)	v-model:active / activator	Current file in sidebar

API Additions

Options

• active: 'single' | 'multiple' — controls how many items can be active (default: 'single')

Context (tree-level)

• activeIds — reactive Set<ID> of active item IDs
• activeItems — computed Set of active item instances
• activeIndexes — computed Set<number> of active item indexes (mirrors selectedIndexes)
• activate(ids) — activate one or more items
• deactivate(ids) — deactivate one or more items
• activated(id) — check if an item is active
• deactivateAll() — clear all active state

Ticket (per-item)

• isActive — reactive ref
• activate() — activate this item
• deactivate() — deactivate this item

Registration

• active: true — set initial active state on register

Behavior

• Single mode: activating an item deactivates all others
• Multiple mode: no limit on concurrent active items
• Cleanup on unregister, clear, and cascade delete
• Zero impact on existing selected/opened state

Checklist

• TypeScript types updated
• Implementation complete
• All 2745 existing tests pass
• Typecheck clean
• Lint clean
• Knip/Sherif clean

[pkg-pr-new]

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commit: ff9f8f2