CI: run the sanitizer gate and broaden major coverage (follow-up to #236)

[ChronicallyJD]

Gap

The two highest-value gates do not run in CI; they are local / honor-system:

• The clang ASAN+UBSAN sanitizer gate (test/run_san.sh / build_san.sh) — the only gate that catches the non-faulting memory-safety class the five assert builds provably miss (it exists because of Fuzz the Parquet decoder, and fix the first defect it found (#214) #225) — runs nowhere in .github/workflows/ci.yml, and its build needs a hand-extracted clang ASAN PostgreSQL from a local tarball. Regressions of exactly that class can land green between manual runs.
• The behaviour suites in CI run on PostgreSQL 17 only (Add a GitHub Actions CI workflow #236); 15/16/18 are build-only and 19 is absent. Any behaviour/WAL/AM-API divergence on the other majors is caught only when someone runs the local five-major matrix by hand.

This builds on #236 (the first CI), which deliberately scoped to build-across-majors + suites-on-one. This issue tracks closing the remaining gap.

Why this bears on a public release

"CI is green" should mean the memory-safety and cross-major behaviour classes are covered. Today it does not. For public confidence the sanitizer gate and broader major coverage need to run automatically, at least on a schedule if too heavy per-PR.

Suggested acceptance criteria

• A CI job (possibly nightly, given cost) that builds the ASAN+UBSAN PostgreSQL and runs run_san.sh.
• Extend suite coverage beyond PG17 (at least add 18; 15/16 as capacity allows). Make any silent SKIP (isolation, pyarrow) fail-loud so "green" cannot mask dropped coverage.

Found in a release-readiness review; follow-up to #236.