json_weights_golden reads priv/algo/band_weights.json via a bare
relative path. That resolves inside `cargo test` at the crate root but
not inside the image build, whose context is rust/prop_grid_rs and
whose WORKDIR is /src — so the test aborted with "JSON file must
exist" and the build never produced an image.
grid-rs CI has been red since 2fd88a94, the commit that added the
test. Production still runs main-1785606663-95976b6 (95976b6b, the
last green build), so every prop-grid-rs change since then — including
the HRDPS rotated-pole decode fix — has silently failed to ship.
Three parts:
* resolve the fixture from CARGO_MANIFEST_DIR so the path no longer
depends on the cwd
* stage the file into the build context in the workflow
* COPY it to /priv in the builder stage, where
CARGO_MANIFEST_DIR/../../priv resolves from /src