RFC 017 aggregate modifiers

[dannymeijer]

Summary

• Adds registry-backed aggregate modifier metadata plus AggregateMeasure.distinct(), .filter(...), and .order_by(...) state.
• Adds count_distinct(expr) and count_if(predicate) compatibility helpers as decorator-registered aggregate rewrites.
• Lowers distinct/filter aggregate modifiers into Substrait, validates invalid modifier shapes, and keeps Prism cross-store adoption sensitive to aggregate modifier state.
• Updates aggregate docs, function catalog docs, release notes, and marks RFC 017 implemented.

Stack / Merge Order

This PR is stacked on #44 and targets feature/33-35-41-wave1-function-catalog.

Merge order:

1. RFC 016/018/024 wave 1 function catalog foundations #44 RFC 016/018/024 Wave 1 function catalog foundations
2. This PR, RFC 017 aggregate modifiers

Verification

• make fmt
• incan test src/functions/aggregates
• incan test tests/test_prism.incn
• incan test tests/test_substrait_plan.incn
• incan test tests/test_session_aggregates.incn
• incan test tests/test_function_registry.incn
• incan test tests/test_dataset.incn
• make fmt-check
• make test-style
• make registry-metadata
• make build
• make test
• make smoke-consumer
• make pre-commit

Closes #34