RFC 019 window functions

[dannymeijer]

Summary

• Completes RFC 019's registry-backed window function surface: window() specs, explicit row/range frame bounds, ranking/distribution helpers, lag/lead, value helpers, and aggregate-over-window placement.
• Threads window applications through Prism, Dataset carriers, Substrait ConsistentPartitionWindowRel lowering, extension declarations, plan inspection, and DataFusion-backed Session execution.
• Preserves output aliases, frame bounds, invocation/null-treatment metadata, and chained window projections through the backend adapter boundary.
• Marks RFC 019 implemented and records PR RFC 019 window functions #48 in the RFC/docs/release notes.

Refs #36.

Verification

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

Final pre-commit result: 159 passed. incan.lock is out of date warning was observed and intentionally not refreshed.