Run the FFI crate's own tests in CI: it is excluded from the workspace, so the
`cargo test --all` in backend-test never reached them and the new guard against
the worker-aborting DECIMAL would not have run. build_dev.sh now honors a
caller-pinned CARGO_TARGET_DIR so the test build reuses that compile instead of
building the bundled engine a second time.
Also pin UHUGEINT rendering, and correct the rust_decimal rationale —
`Decimal::new` is public without the feature, so the reason is that the feature
reproduces the exact binding the crate used to derive, not that nothing else can.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Ruben Fiszel
5cad843e4e