Muse reported that find({ orderBy: 'createdAt' }) silently returned the
wrong order: getFieldValueForEntity read noun.metadata[field] for
timestamp fields, but getNoun() destructures standard fields to the top
level, so the read always returned undefined and the sort collapsed to
insertion order.
The fix introduces a single source of truth for reading fields off an
entity. STANDARD_ENTITY_FIELDS + resolveEntityField() in coreTypes.ts
encode the "standard fields top-level, custom fields nested" contract
in one place. getFieldValueForEntity now uses the helper and routes
through a named BUCKETED_INDEX_FIELDS set instead of a hardcoded
timestamp if-chain — filtered sort on createdAt/updatedAt now works.
Unfiltered orderBy is explicitly rejected with a clear error pointing
callers at the right pattern. A scalable, unfiltered-sort-capable
time-ordered segment index is tracked as a separate follow-up.
