Add `item.chain` getter

[nuxodin]

Similar to path (which returns an array of keys), chain would return an array of the actual item instances from root to the current item.

get chain() {
    if (this.#parent == null) return [this];
    return [...this.#parent.chain, this];
}

Difference to path:

item.path   // ['users', '42', 'name']  – array of keys (strings)
item.chain  // [rootItem, usersItem, item42, nameItem]  – array of Item instances

Useful when you need to traverse or inspect the ancestor items themselves, not just their keys.

Naming alternatives

Before implementing, worth considering alternative names:

Name	Notes
chain	current proposal, known from method chaining / Promise chains
breadcrumbs	very intuitive for "path from root to here", common in UI navigation
trace	known from stack traces
lineage	descriptive but uncommon in JS APIs
ancestors	slightly misleading as it would also include the item itself

Leaning towards chain (consistent with existing ecosystem) or breadcrumbs (most intuitive semantically). Thoughts?