[M1] ShardedStreamer: shard-filtered NEAR Lake wrapper

[Fosurero]

Description

Implement ShardedStreamer that wraps near-lake-framework and filters StreamerMessage by shard_id.

Tasks

• ShardedLakeConfig with network, start_block, shard_ids
• ShardedStreamer::new() constructor
• build_lake_config() for real NEAR Lake connections
• should_include_shard() filter predicate
• stream_demo() synthetic data generator
• streamer_message_to_shard_blocks() conversion function
• Unit tests

Status: ✅ Complete – M1 PoC