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.
• Merged with current origin/main after RFC 016/018/024 wave 1 function catalog foundations #44 and removed the same redundant-helper/source-quality smells found during the RFC 016/018/024 wave 1 function catalog foundations #44 review loop.

Merge Order

#44 has been merged. This PR now targets main directly and can be reviewed as the RFC 017 follow-up.

Verification

• make fmt INCAN=/Users/danny/Development/encero/incan/target/debug/incan
• /Users/danny/Development/encero/incan/target/debug/incan test tests/test_function_registry.incn
• /Users/danny/Development/encero/incan/target/debug/incan test tests/test_substrait_plan.incn
• /Users/danny/Development/encero/incan/target/debug/incan test tests/test_session_aggregates.incn
• /Users/danny/Development/encero/incan/target/debug/incan test tests/test_dataset.incn
• /Users/danny/Development/encero/incan/target/debug/incan test src/functions/aggregates
• make pre-commit INCAN=/Users/danny/Development/encero/incan/target/debug/incan

Closes #34