feat(data): add Observe.fromKeys reactive keyed-collection combinator

[krisnye]

Description

Adds Observe.fromKeys — a reactive keyed collection combinator:

fromKeys<K, V>(keys: Observe<readonly K[]>, observeChild: (key: K) => Observe<V>): Observe<readonly V[]>

Given an observable list of keys and a factory mapping each key to a child observe function, it yields the child values in key order. Child subscriptions are keyed by === identity and reused across list changes — only added keys create a child and only removed keys dispose one — so a value change on one child re-emits only that child's slot (siblings keep their values, no recompute, no re-subscribe). It emits once every current child has yielded a value (synchronously when all children are synchronous), and re-emits on any key-list change or child emission.

This is the dynamic, keyed counterpart to the existing fromArray / fromProperties, which combine a fixed set of observe functions; here the set of children is itself driven by an observe function of keys.

Motivation

It generalizes a local fromEntities helper written for firefly-platform's avTimelinePayload reactive-graph refactor (select the ordered entity set → one per-entity child observable → collect), so that consumer can drop the private helper and import this instead. It is the "membership set → per-entity leaf" substrate for building O(changes) reactive dependency graphs over an ECS.

Tests

New from-keys.test.ts (peer to the source): ordered emission, per-slot re-emit with sibling reuse (asserts sibling subscription count is untouched), key add / remove (disposes child) / reorder, waiting for all children before first emit, empty-list, and full disposal (keys + every child unsubscribed).

pnpm run typecheck, pnpm run lint, and the full observe suite (176 tests) pass. Version bumped to 0.9.80 via pnpm run bump.