unique_key_* session settings

These settings are available in system.settings and are autogenerated from the ClickHouse snapshot.

unique_key_max_encoded_size

Maximum size (in bytes) of the order-preserving binary encoding of a single UNIQUE KEY row.

unique_key_probe_implementation

Selects the UNIQUE KEY write-path probe implementation. Currently only a single-threaded baseline exists, so auto and simple both resolve to it.

Reserved: this setting has no effect yet — the value is not read until the UNIQUE KEY INSERT write path that constructs the probe is wired up. Declared now so the lever ships with the implementation.