Jianghua yang: Pricing plans in PostgreSQL's currency: pg_orca's PG-aligned cost model

How pg_orca replaced ORCA's Greenplum-calibrated cost model with a port of PostgreSQL's costsize.c: where it plugs in, how it bridges ORCA's total-only costs and rebinds to PostgreSQL's startup costs and loop counts, and how a regression harness keeps both planners within 2%.