0.12.0-rc.2

PostgreSQL 16-19 EXPLAIN text-rendering fidelity.

pkg/pgexplain now byte-reproduces psql's native FORMAT TEXT from a JSON plan:
- fix(pgexplain): aggregate Strategy/Partial labels (GroupAggregate/MixedAggregate,
  Finalize/Partial), never-executed nodes, trigger for-constraint, Function Scan
  alias + schema-qualification, comma-separated Buffers sections, Bitmap Index Scan 'on'.
- feat(pgexplain): render previously-dropped fields - Memoize cache stats, Join Filter
  + Rows Removed, Incremental Sort Presorted Key + Full-sort/Pre-sorted Groups, Index
  Recheck + Heap Blocks, HashAgg spill, WindowAgg Run Condition.
- feat: cmd/explainrender CLI to render EXPLAIN JSON via joe's renderer.

All red/green against real PostgreSQL 16/17/18/19beta1; oracle is psql FORMAT TEXT.
Audit + remaining strict-re-diff backlog tracked in #210.