fix: one field-resolution law across aggregation hooks, source.where, removeMany, and find() spellings

Four fixes from a consumer conformance report, one root disease — two
field-resolution regimes where there must be one:

- The delete/update aggregation hooks fed the engine a partial entity
  view (type/service/data/metadata only), so a reserved-field groupBy
  (subtype, visibility, ...) resolved to a nonexistent group on the way
  down: counts drifted upward forever after deletes, and updates moving
  an entity between reserved-field groups double-counted. The hooks now
  pass the full-fidelity view via entityForAggFromRawRecord (every
  reserved field top-level, mirroring the add path); the update sites
  pass the full get() view instead of a hand-rolled subset.
- Aggregation source.where resolved fields only against the custom
  metadata bag, so where on a reserved field silently matched nothing.
  The matcher now resolves each filtered field through
  resolveEntityField — the same single source of truth groupBy uses.
- removeMany() with no usable selector (bare array passed positionally,
  empty params, ids: []) resolved successfully having deleted nothing.
  All three now throw; the two legacy tests that pinned the silent
  no-op as 'graceful' now pin the refusal.
- find() where keys accept both spellings: a metadata.-prefixed key
  falls back to its flattened spelling when the prefixed one is not
  indexed (metadata is flattened at index time). A literal nested custom
  key named metadata still wins when indexed as spelled.

Five regression pins in aggregate-reserved-fields.test.ts (4 of 5 vary
red on the unfixed code).

tests/unit/brainy/batch-operations.test.ts

@@ -533,8 +533,10 @@ describe('Brainy Batch Operations', () => {
       expect(result.successful).toHaveLength(0)
       
       await brain.updateMany({ items: [] })
-      await brain.removeMany({ ids: [] })
-      // Should not throw
+      // removeMany is the exception (8.8.2): an empty id list is a refused
+      // selector, not an empty batch — deleting "nothing" silently was the
+      // bug class (a positional/bare-array call looked identical).
+      await expect(brain.removeMany({ ids: [] })).rejects.toThrow(/ids: \[\]/)
     })
     
     it('should validate batch size limits', async () => {