Context

Several operators in this ticket have non-trivial Go design considerations:

• $switch requires a new SwitchCase helper type (for branches) to be added alongside the operator. Model it after the SetField/setField sealed-interface pattern if appropriate, or as a simple exported struct — pick whichever is cleaner.
• $let's vars argument maps variable names to expressions. Use map[string]any for the vars parameter, but note in a code comment that Go map iteration is non-deterministic, so tests must construct the expected bson.D with a fixed field order rather than round-tripping through a map.
• $percentile and $median require a method argument that currently must always be "approximate". Hardcode it and leave a TODO comment for when other methods are supported.

Definition of Done

• All of the following operators are implemented in mql/agg/operator.go, each with a one-line doc comment:

• • Conditional: $cond, $switch (plus exported SwitchCase type and Case constructor)
  • Higher-order: $let, $map, $reduce, $function
  • Misc: $literal, $rand, $meta, $createObjectId
  • Statistical: $percentile, $median
  • Similarity: $similarityCosine, $similarityDotProduct, $similarityEuclidean
  • Hash/EJSON: $hash, $hexHash, $serializeEJSON, $deserializeEJSON
• Each operator has at least one test in mql/agg/operator_test.go