Upgrade to pgrx 0.19.1 — PostgreSQL 18 (resolve E-006)

[styk-tv]

Closes #63. Resolves ERRATA E-006 (see new E-015 in specs/ERRATA.v0.6.md).

What this does

Upgrades pgrx 0.16.1 → 0.19.1 and moves the active PostgreSQL target to 18.
pgRDF v0.6.19 builds, links, boots and runs on PostgreSQL 18.4 with no
functional code changes — only the mechanical 0.18 migration.

Why E-006 no longer holds

The two blockers that held pgRDF at pgrx 0.16.1 were point-in-time, and the
errata's own re-check trigger ("any pgrx publish above 0.18.0") had fired
(0.18.1, 0.19.0, 0.19.1 shipped):

• E0658 (NonNull::from_mut) is a Rust-version gate — clears on Rust ≥ 1.96,
  which pgrx adopted as its MSRV in 0.18.1.
• E0716 (impl_table_iter) clears via pgrx 0.18's SqlTranslatable →
  associated-const migration (the &[..] temporary now gets 'static
  promotion in const context).

Changes

Extension

• Cargo.toml: pgrx/pgrx-tests → =0.19.1; add pg18 feature; check-cfg
  known values → pg13,pg19.
• Remove [[bin]] pgrx_embed + delete src/bin/pgrx_embed.rs (0.18 single-pass
  schema embed). pgRDF has zero hand-written SqlTranslatable impls, so the
  migration's main breaking change doesn't touch us. crate-type keeps "lib".

Local build / compose

• compose/builder.Containerfile: rust:1.91 → 1.96, PGRX_VERSION → 0.19.1.
• compose/compose.yml: postgres:18-bookworm, /18/ extension paths, and PGDATA
  on a named volume mounted at /var/lib/postgresql (PG18's data-dir convention;
  Upgrading between major versions? docker-library/postgres#37).

CI / release — moved active major 17 → 18, preserved 17 (armed, not run)

• PGRX_VERSION → 0.19.1 everywhere (must match the new pin or even pg17 breaks).
• ci.yml clippy + test matrices → ["18"]; compose regression job → pg18.
• release.yml, oci-publish.yml, update-latest-md.yml (attested chain) → pg18.
• pg17 and pg14–16 are kept in the commented full matrices — one active major, no
  CI/artifact-credit bloat.

Validation

• Local: clean --features pg18 build (0 errors/0 warnings), cargo fmt --check
  • clippy -D warnings green, and a live pgrdf-compose stack on PG 18.4:
    CREATE EXTENSION pgrdf + pgrdf.version() = 0.6.19 + a SPARQL load_turtle→join
    round-trip.
• Pipeline: draft CI runs fmt + clippy(pg18); marking Ready runs the full pg18
  cargo pgrx test + compose regression + W3C harnesses.

Not in this PR (release-cut / follow-up)

• Full pg18 cargo pgrx test + regression green in CI (flip to Ready)
• Version bump + CHANGELOG fold happen at the release cut (v0.6.20)
• PG 19 follow-up (add pg19 feature + postgres:19; pgrx 0.19.x already
  carries pg19beta, but PG 19 is still beta — not for an attested cut yet)

Kept as a draft pending the full pg18 pipeline pass + the maintainer's merge/
release call.