Add a Mendix 8 runtime crate

[hjonck]

The crate family covers Mendix 7, 9, 10, and 11 — Mendix 8 is the one gap.

What's needed

1. A CDN-available MX8 runtime version. The build pulls https://cdn.mendix.com/runtime/mendix-<version>.tar.gz; we have not yet found a working MX8 URL (probes of 8.18.x returned 404 — the exact patch/build string is unknown). Finding a real, pullable MX8 version is step one.
2. A new crates/mendix-8/ — copy crates/mendix-7/ (MX8 is also Java 8), set ARG MENDIX_VERSION + the FROM eclipse-temurin:8-jre-jammy base, and update the README/versions.yaml.
3. Smoke-test against a real MX8 app (tests/smoke-test.sh <unzipped-mda>) and record the result in versions.yaml + provenance.yaml.

See CONTRIBUTING.md and the Mendix 7 guide for the pattern. Recipe-only — no Mendix binaries (the CI guard enforces this).

[hjonck]

Done — a Mendix 8 runtime crate (plus a build crate) shipped in #9 and is now on main: crates/mendix-8.

Two notes:

• MX8 turned out to be CDN-available after all. The final 8.18 LTS patch (8.18.35.97) — runtime and mxbuild — is served from cdn.mendix.com (HTTP 200), so the crate is a normal recipe-pull, no portal login needed. Older MX8 patches the CDN no longer serves are covered by PORTAL-DOWNLOAD.md.
• The recipe + CDN source are verified, but a full boot-smoke against a real MX8 app (HTTP 200, like the MX7/9/10/11 fixtures) is still open — provenance.yaml records smoke_verified: pending. If you maintain an MX8 app and can confirm it boots on the crate, a PR adding the fixture row to crates/mendix-8/versions.yaml is very welcome.

Closing as the crate now exists; community help on the verified fixture stays open via #4.