Add support for FINAL when joining tables

[cbisnett]

Summary

ClickHouse's final modifier only wraps the query's primary FROM source, so .final.joins(:assoc) emits a plain INNER JOIN <table> against the un-merged joined table (wrong row mapping, duplicates). This adds two query methods that render FINAL on the joined table:

AppControlEvent.final.joins_final(:binary)
# SELECT ... FROM app_control_events FINAL
#   INNER JOIN binaries FINAL ON ...

AppControlEvent.left_joins_final(:binary)
# SELECT ... FROM app_control_events
#   LEFT OUTER JOIN binaries FINAL ON ...

joins_final adds the association like joins (INNER JOIN); left_joins_final adds it like left_outer_joins (LEFT OUTER JOIN) for optional joins whose rows must be preserved when there is no match — while still merging the joined table with FINAL.

Implementation

• Arel::Nodes::FinalTable — a new Unary marker node wrapping a join's table source. Because every Arel join visitor (InnerJoin, OuterJoin, …) emits its table via visit o.left, a single visit_Arel_Nodes_FinalTable covers all join types.
• Relation#joins_final / #left_joins_final (+ ! variants) add the join(s), record the requested association names in joins_final_values, and — in build_arel, before the FROM-level FINAL is applied — wrap matching join sources in FinalTable.
• Joins are matched by table name (handles Arel::Table and TableAlias). If the same physical table is joined more than once, every join of that table receives FINAL (documented on the methods).
• Class-level delegation (Model.joins_final), chaining, dedup with an existing .joins, and unscope(:joins_final) are all supported.
• Raises ActiveRecord::ActiveRecordError on non-ClickHouse connections, consistent with the existing final/settings clauses.

Changes

• lib/arel/nodes/final_table.rb — new marker node
• lib/arel/visitors/clickhouse.rb — visit_Arel_Nodes_FinalTable
• lib/active_record/connection_adapters/clickhouse_adapter.rb — require the node, delegate the new methods
• lib/core_extensions/active_record/relation.rb — the query methods, join-marking, and unscope support
• README.md, CHANGELOG.md — docs
• spec/single/model_spec.rb — specs covering generated SQL, FINAL combination with .final, dedup, unscope, INNER-vs-LEFT, mixed same-association use, and real execution against ClickHouse (FINAL requires a supporting engine, so the execution test self-joins the sample table)