Split bindings.rb into per-area files

[coipond-coder]

Redmine: 47594

Mechanical split of lib/rdkafka/bindings.rb (810 lines) into lib/rdkafka/bindings/*.rb. No behaviour or API change.

• Every file reopens the same Rdkafka::Bindings module (no submodules). ffi_lib, typedefs and callback constants stay on one module, so Rdkafka::Bindings.rd_kafka_* and the karafka/karafka-core patches (remove_const/const_set on callbacks, external attach_function) keep working.
• bindings.rb keeps ffi_lib + glibc error, shared constants, SizePtr, rd_kafka_global_init, then require_relatives: structs (all FFI::Structs, loaded first), polling, metadata, messages, configs, list_offsets, errors, configuration, callbacks, handle, consumer, producer, admin, acl.
• Lines moved verbatim. A sorted-line diff of old vs new shows only the wrapper lines, the require_relatives, and 2 comments (# Stats → # Watermark offsets, new # Messages, topics and topic partition lists header). Moves across sections: the structs into structs.rb, rd_kafka_query_watermark_offsets into list_offsets.rb, rd_kafka_group_result_* into admin.rb.
• Parity vs master b4ca546 (same machine): 275 singleton methods (names + arities), 117 constants (struct layouts and sizes included), 8 typedefs, 2 enums: identical. spec/lib/rdkafka/bindings_parity_spec.rb pins those 275 methods / 117 constants (additions need no spec change) and checks the 5 callback constants are own constants.
• .rubocop.yml / .yard-lint.yml: the bindings.rb exclusions also cover lib/rdkafka/bindings/**/*.

Overlap: #1019 adds 3 rd_kafka_message_* bindings and RD_KAFKA_MSG_STATUS_* after rd_kafka_message_timestamp. Whichever merges second moves them into lib/rdkafka/bindings/messages.rb.