feat(sql): plan joins over summary children

[zzylol]

Why

A SQL join was retained as one KeepPreAsap subtree, so temporal aggregate children on each side could not independently bind to reusable summaries. For example, joining two grouped asap_rate(...) subqueries forced the complete join through exact execution.

What

• Add a language-neutral SummaryExpr::RelationalJoin to the post-ASAP IR and executable DAG.
• Recursively plan both sides of supported inner column-equality joins.
• Preserve join kind, predicate, grouping boundaries, output schema, and strict execution data-state validation.
• Keep unsupported outer joins and predicate shapes as local exact residuals.
• Finalize exact accumulator state into the canonical readable schema before query-time composition.
• Export the whole-graph validator's authoritative edge state instead of deriving it twice.

How

Bottom-up binding plans each join input independently, then emits a relational node only when both inputs and the predicate satisfy the typed contract. DAG validation checks operator payloads, schemas, grouping, and execution phases before export. Exact accumulator finalization restores the original pre-ASAP value types, so downstream joins and aggregates consume rows rather than opaque state.

Before

A join over two summary-capable temporal aggregates remained one exact subtree.

After

Each temporal aggregate can bind independently, and the read-time join composes their typed results in the same post-ASAP DAG. Unsupported children remain exact without replacing supported siblings.

Verification

• cargo fmt --all -- --check
• cargo test -p asap-types post_asap::executable_dag
• cargo test -p asap-integration-tests --test promql_to_post_asap
• Full workspace CI