`find({ where })` had two gaps that could silently return nothing. The in-memory
matcher (`matchesQuery`, used for egress re-validation and historical reads) was
missing several documented operators — `in`, `greaterThanOrEqual`,
`lessThanOrEqual` — so it disagreed with the index path, which does handle them.
And an unknown operator key (a typo, or `notIn` written where `not: { in }` was
meant) fell through to a nested-object-field interpretation and matched nothing.
- Align `matchesQuery` to the full documented operator set (add the missing
aliases; the default branch now throws instead of treating an unknown key as a
nested field — dotted paths remain the supported nested form).
- Validate the `where` filter up front (`validateWhereFilter`), throwing a typed
`BrainyError('INVALID_QUERY')` that names the unknown operator, recursing into
`allOf` / `anyOf` / `not`.
- Stop excluding a user field literally named `level` from the metadata index
(it collided with an internal never-index key), so numeric/exact filters on it
resolve instead of returning 0.
Adds unit coverage for the operator set and the throw-on-unknown behavior.
