feat(ecs): add reactive observe(arg) to index handles

[krisnye]

Description

Index handles (db.indexes.<name> / t.indexes.<name>) gain a reactive method:

db.indexes.orderedChildrenOf.observe(parent) // → Observe<readonly Entity[]>

observe(arg) is the reactive counterpart to find(arg). It emits the current sorted bucket synchronously on subscribe, then re-emits — on a microtask after a committed transaction — whenever that bucket's membership or order changes, and stays silent for transactions that touch only other buckets.

Why

Previously a reactive sorted view required pairing db.observe.select(..., { where, order }) with db.indexes.find(). That's a correctness footgun: a sort-key-only reorder changes the index's order but not the where result, so observe.select silently swallows it. observe reports it.

Design note (deviates from the original spec)

The original plan proposed notifying subscribers synchronously inside the index's add/remove/update. Those run per-mutation, mid-transaction (via applyInsert/Update/Delete), before commit — which would fire observers re-entrantly against half-applied state, leak intermediate states on multi-mutation transactions, and run at a different cadence than every other observer (observe.select, observe.entity, … all fire once at the execute → notifyObservers commit boundary).

Instead this mirrors the established observeSelectEntities:

• New observe-index-entities.ts subscribes to observe.transactions, gates on changedComponents ∩ readColumns, debounces via queueMicrotask, and recomputes through the index's own find(arg). Bucket precision + reorder detection both fall out of comparing the recomputed sequence to the last emitted one.
• observe is attached at the database layer (where observe.transactions lives); db.indexes and t.indexes share handle objects, so both are covered.
• No changes to the index's per-mutation maintenance logic (lower risk).

Changes

• observe-index-entities.ts — new reactive helper.
• index-types.ts — observe(arg): Observe<readonly Entity[]> on Index.Handle.
• create-store.ts — exposes the index's readColumns on the handle (internal, like routableColumns).
• create-database.ts — attaches handle.observe.
• Tests: 16 runtime tests + a type-test + README docs.

All 122 index tests, workspace typecheck (tsc -b), and lint of touched files pass.

Related PRs

• Builds on feat(ecs): pluggable concurrency strategies for database #113, Add typed, plugin-declared indexes to the ECS database #107 (ECS indexes).

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