feat: opt-in FK cascade retain for row_filters (#88)

[ababic]

Summary

Implements #88: optional parent→child retain cascading for row_filters.

[row_filters."public.listing_order"]
retain = [{ column = "status", op = "eq", value = "open" }]

[[row_filters."public.listing_order".cascade]]
child_table = "public.listing_orderitem"
child_fk = "order_id"
parent_pk = "id"

While streaming a parent table, Dumpling records parent_pk values of kept rows. Child rows are kept only when child_fk is in that set (in addition to any local retain/delete). NULL FKs are dropped.

Design notes

• Opt-in and shallow (explicit cascade links only)—no live FK discovery or full graph topo sort (per issue non-goals).
• Single-pass streaming: parent data must appear before child data; otherwise Dumpling fails with a clear error.
• Retained PK sets are held in memory for the run.

Status

Rebased onto latest main (includes datetime/date predicate comparisons from #87 / #89).

Verification (post-rebase)

• cargo test --all-targets --all-features — 182 passed
• cargo fmt --all -- --check — clean
• cargo clippy --all-targets --all-features -- -D warnings — clean