feat: add batchFind* methods to edge collections for store.batch()

[pdlug]

• Add batchFindFrom, batchFindTo, and batchFindByEndpoints to edge collections, returning BatchableQuery for use with store.batch()
• Add compile-time type assertions (source-level @ts-expect-error + tsd declaration tests) verifying endpoint constraints and return types
• Update docs: schemas-stores, queries/execute, performance/overview

Motivation

Edge collection lookups (findFrom, findTo, findByEndpoints) execute immediately and can't participate in store.batch(). This forces users to rewrite simple edge lookups as verbose fluent traverse queries to batch them. The new batchFind* variants close this gap.

const [skills, employer] = await store.batch(
  store.edges.hasSkill.batchFindFrom(alice),
  store.edges.worksAt.batchFindFrom(alice),
);

Closes #51