RFC 019 window function foundations

[dannymeijer]

Summary

• Adds RFC 019's first window-function foundation slice: window(), row_number(), rank(), dense_rank(), and with_window_column(...).
• Threads window projections through Prism, Dataset carriers, Substrait lowering, extension declarations, plan inspection, docs, and release notes.
• Marks RFC 019 as In Progress and keeps remaining value/offset/frame/query-syntax work explicit.

Scope

This PR intentionally does not implement full RFC 019. It does not add DataFusion execution for window relations, lag/lead, value functions, aggregate-over-window, frames/null treatment, or query-block OVER (...) syntax.

Stack order: merge PR #44, then #45, then #46, then #47, then this PR.

Refs #36.

Verification

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

Final pre-commit result: 142 passed.