Decodes ClickHouse's plaintext "Native" block format -- the payload of a Data (or wire-identical ProfileEvents) packet once any compression envelope has been stripped away.

Compression is opt-in and defaults to off (ChDriver.Protocol.encode_query/2's :compression opt, :none by default). When it's off, blocks arrive exactly as described below. When a query negotiates :lz4 compression, decode_data_packet/2 routes the block body through ChDriver.Protocol.Block.Compressed.decode/1 first and hands this module the decompressed bytes -- the format below is unchanged either way, since compression is just an envelope around the same plain Native block bytes.

Format (matches Formats/NativeReader.cpp, protocol revision 54469):

external_table_name (string -- only present when this block is the
  payload of a Data/ProfileEvents *packet*; see `decode_data_packet/1`)
BlockInfo:
  repeated (field_num varint, value) pairs, terminated by field_num 0
    field 1 -> is_overflows (UInt8)
    field 2 -> bucket_num (Int32, little-endian)
num_columns (varint)
num_rows (varint)
num_columns times:
  name (string)
  type (string)
  has_custom_serialization (UInt8 -- gated on
    DBMS_MIN_REVISION_WITH_CUSTOM_SERIALIZATION, always present at
    revision 54469; we only support has_custom == 0)
  column data (num_rows values, encoding depends on type -- see
    `decode_column_data/3`)

Only a pragmatic subset of ClickHouse's type system is supported -- enough to prove out SELECT 1 / SELECT number FROM system.numbers and to skip over the columns ClickHouse's own ProfileEvents packet sends alongside every query result (String, DateTime, UInt64, Enum8, Int64).

This module owns block/packet framing and the top-level decode_column_data/3 type dispatch only. The rest of the type system is split out by concern, matching how a new type or wrapper should be added:

• ChDriver.Types -- parses a column type string (e.g. "Nullable(String)") into its component parts. Add a new parse_*/1 clause here for a new wrapper syntax.
• ChDriver.Types.Registry -- the scalar/fixed-width codec table (column_codec/1) and the primitive wire readers (decode_fixed_width/4, decode_strings/3) it hands out. Add a new scalar ClickHouse type here.
• ChDriver.Protocol.Block.Wrappers -- decoders for Nullable(T), Array(T), Map(K, V), LowCardinality(T), and Decimal(P, S). Add a new wrapper/compound decoder here, alongside a dispatch clause in this module's decode_column_data/3 and a parser in ChDriver.Types.
• ChDriver.Protocol.Block.Sparse -- decodes MergeTree's "sparse" column serialization (see its moduledoc for the full wire format).

decode_column_data/3 is this module's extension point for new wrapper/compound types: add a clause here that pattern-matches/parses the type string via ChDriver.Types, plus a decode_* function in ChDriver.Protocol.Block.Wrappers. Note that decode_column_data/3 recurses through ChDriver.Protocol.Block.Wrappers and ChDriver.Protocol.Block.Sparse (that's what makes Array(Nullable(String)) work for free), so those modules call back into this one across the module boundary -- deliberately fine in Elixir, since none of the calls are macros.