fix: preserve $meta direction in [field, direction] sort pair

[spokodev]

Description

Sorting by $meta is silently rejected in the [field, direction] pair form of sort. The documented, type-legal input

cursor.sort(['score', { $meta: 'textScore' }])

which is the canonical way to sort full-text search results by relevance, throws:

MongoInvalidArgumentError: Invalid sort direction: [{"$meta":"textScore"}]

The equivalent object form { score: { $meta: 'textScore' } } and the deep form [['score', { $meta: 'textScore' }]] both work, so the three shapes the API accepts behave inconsistently.

Root cause

In src/sort.ts, pairToMap calls prepareDirection([v[1]]), wrapping the direction in an array, unlike every sibling converter which passes the raw value. A scalar direction coincidentally survives the array to string coercion ([-1] -> "-1"), but a $meta object does not (isMeta returns false for an array), so it is rejected.

This is a regression: $meta-in-pair support was added in #1808, and the #2573 sort refactor reintroduced the array wrapping.

Fix

-  return new Map([[`${v[0]}`, prepareDirection([v[1]])]]);
+  return new Map([[`${v[0]}`, prepareDirection(v[1])]]);

Test

Added test/unit/sort.test.ts (no test/unit file covered formatSort before). The $meta pair case fails unpatched with the error above and passes after; the scalar-direction pair cases pass throughout, showing the fix is scoped. Full test/unit suite stays green.