0.12.0-rc.3

PostgreSQL 16-19 EXPLAIN text-rendering fidelity — long-tail closure + CI guard.

Builds on 0.12.0-rc.2. pkg/pgexplain now byte-reproduces psql's native FORMAT TEXT
across PG16-19, enforced by a per-version CI fidelity guard.

- fix(pgexplain): 8 byte-fidelity fixes found by a strict joe-vs-psql re-diff —
  suppress 'Rows Removed by Filter: 0'; SetOp/HashSetOp label; Function Scan
  'Function Call:'; multi-function ROWS FROM alias; Result 'One-Time Filter';
  Tid Scan 'TID Cond'; quote_identifier for special Subquery Scan aliases;
  PG18+ Storage line ordering.
- test(pgexplain): per-version CI fidelity guard (TestFidelityAgainstLiveServer +
  fidelity-pg-matrix jobs) asserting joe == psql FORMAT TEXT after a documented
  4-trait normalization; small allowlist = the known #210 gaps.

Reviewed by akartasov. Audit + remaining backlog tracked in #210.