Handles Redis HyperLogLog commands: PFADD, PFCOUNT, PFMERGE.

Each handler takes the uppercased command name, a list of string arguments, and an injected store map. Returns plain Elixir terms -- the connection layer handles RESP encoding.

HyperLogLog sketches are stored as plain 16,384-byte binary values in the store with no expiry (expire_at_ms = 0). They are transparent to the store layer -- just another binary blob keyed by a string.

Supported commands

• PFADD key element [element ...] -- adds elements to the HLL sketch at key, creating it if absent. Returns 1 if the sketch was modified, 0 if not.
• PFCOUNT key [key ...] -- returns the estimated cardinality. For a single key, returns the estimate from that sketch. For multiple keys, merges sketches in memory (without writing) and returns the combined estimate. Non-existent keys are treated as empty sketches.
• PFMERGE destkey sourcekey [sourcekey ...] -- merges all source sketches into destkey (which may or may not already exist). For each register position, takes the maximum value across all sources and the existing dest. Returns :ok.