fix(sql): preserve nested temporal aggregates

[zzylol]

Why

An outer SQL aggregate could hide an otherwise reusable inner temporal aggregate, forcing the complete nested expression through exact execution.

What

• Preserve nested temporal aggregate structure during SQL lowering.
• Bind the inner temporal summary independently.
• Keep the outer aggregate as an explicit read-time operation with strict schema and execution-state validation.

How

Recursive lowering retains the inner aggregate node instead of flattening its output into an opaque leaf. The outer operator consumes the finalized readable values from that child, preserving the maintenance/read boundary.

Before

SUM(asap_rate(...)) became one exact residual subtree.

After

The rate state can be precomputed and reused, while the outer sum executes over its readout in the same DAG.

Verification

• cargo fmt --all -- --check
• cargo test -p asap-integration-tests --test sql_to_post_asap (11/11)
• Full workspace CI