feat(common): initial multi-network incrementalizer support

[Theodus]

Introduce lead-network incrementalization for streaming SQL queries spanning
multiple blockchain networks, interpolating non-lead _block_num values into
the lead network's block range using segment metadata.

• Add IncrementalContext with single_network/multi_network constructors
  and interpolate_block_num_from_segment projection for non-lead tables
• Extract validate_table_providers into a standalone plan walk and
  build_table_network_map from find_cross_network_join for reuse
• Add Filter node above constrained table scans for providers without
  filter pushdown (e.g. MemTable)
• Parse SETTINGS lead_network = '...' in helpers.rs and wire through
  flight.rs → StreamingQuery::spawn
• Add 5 incrementalizer unit tests + 3 lead_network_setting parser tests
• Cross-network joins are not yet supported; when added, the lead network's
  _block_num will pass through to the output table directly

[Theodus]

Properly resolving the set of networks associated with derived tables will be added in another PR.

[Theodus]

This PR does not attempt to infer which network is "fastest" using the segment metadata because:

• Different networks may be faster or slower across segments, if for example the faster chain pauses.
• This makes it clear to the user which network will be chosen, and gives them a setting to make the choice themselves.