RFC 016/018/024 wave 1 function catalog foundations

[dannymeijer]

Summary

This is the base PR for the function-catalog stack.

• RFC 016: add registry-backed core aggregate helpers for avg, min, and max; preserve sum/count(); add count_expr(expr) as the expression-count compatibility path.
• RFC 018: add a narrow common scalar math slice: abs, ceil, floor, and round.
• RFC 024: add function policy/namespace metadata for portable core, extension-only, compatibility alias, engine-specific, and rejected categories.
• Update Substrait function-extension lowering, aggregate relation lowering, registry inspection helpers, docs, and session tests.

Count expression note

The current decorator/static metadata lowering does not yet support an exact decorated helper spelling for both count() and count(expr). I tested default-argument and variadic decorated helper shapes; both failed at compile/lowering boundaries. This PR keeps count() canonical and exposes count_expr(expr) as a compatibility helper that lowers through canonical Substrait count metadata.

Verification

• make fmt
• make fmt-check
• make test-style
• make registry-metadata
• make build
• make test
• make smoke-consumer
• make pre-commit

Refs #33
Refs #35
Refs #41