feat(repl): SQL filter() with WHERE/HAVING, FilterSpec, and dual straddling filters

[aisrael]

Summary

Adds REPL filter("...") for DataFusion SQL predicates, with placement before or after select (and optional group_by). Supports two filter stages when they straddle select(...) for WHERE-like plus HAVING-like predicates in one pipeline. Introduces FilterSpec as a newtype for filter SQL strings on the pipeline builder.

Key changes

• REPL: Parse and validate filter; up to two filters only if one is strictly before and one strictly after select; docs in docs/REPL.md.
• Pipeline: filter_before_select / filter_after_select as Option<FilterSpec>; finalize_dataframe_source applies before → select/aggregate → after.
• Breaking (programmatic): Replaces older single-filter API; callers set one or both filter options explicitly (REPL bridge maps stages).

Verification

cargo +nightly fmt, cargo clippy --all-targets -- -D warnings, cargo test pass locally.