fix(ci): stage band_weights.json into the prop-grid-rs build context

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

.forgejo/workflows/build-grid-rs.yaml

@@ -79,6 +79,14 @@ jobs:
 
           echo "$REGISTRY_TOKEN" | docker login "$REGISTRY" -u "$OWNER" --password-stdin
 
+          # The build context is the crate dir, but the json_weights_golden
+          # test compares against priv/algo/band_weights.json at the repo
+          # root. Stage it into the context so `cargo test` inside the image
+          # can reach it; without this the test aborts and no image is
+          # produced (grid-rs CI was red from 2fd88a94 onward).
+          mkdir -p rust/prop_grid_rs/priv/algo
+          cp priv/algo/band_weights.json rust/prop_grid_rs/priv/algo/
+
           DOCKER_BUILDKIT=1 docker build \
             --file rust/prop_grid_rs/Dockerfile \
             --build-arg BUILDKIT_INLINE_CACHE=1 \

rust/prop_grid_rs/Dockerfile

@@ -115,6 +115,12 @@ COPY Cargo.toml Cargo.lock* ./
 COPY src ./src
 COPY tests ./tests
 
+# `json_weights_golden` compares the compiled-in band weights against
+# priv/algo/band_weights.json, which lives outside this build context.
+# The workflow stages it into the context first; WORKDIR is /src, so
+# CARGO_MANIFEST_DIR/../../priv lands at /priv.
+COPY priv /priv
+
 RUN --mount=type=cache,target=/usr/local/cargo/registry,sharing=locked \
     --mount=type=cache,target=/usr/local/cargo/git,sharing=locked \
     cargo clippy --release --all-targets -- -D warnings \

rust/prop_grid_rs/tests/json_weights_golden.rs

@@ -98,12 +98,20 @@ fn assert_weights_equal(label: &str, got: &Weights, expected: &Weights) {
 /// threads for `#[test]` functions in this binary.
 static ENSURE_ENV: std::sync::Once = std::sync::Once::new();
 
+/// Absolute path to the repo's calibration JSON, resolved from the
+/// crate root rather than the cwd. `cargo test` runs with the cwd at
+/// the crate root, but the Docker build stages the file at an absolute
+/// path, and a bare relative path silently resolved to a nonexistent
+/// file there — which is what turned this golden test into a hard CI
+/// failure that blocked every prop-grid-rs image build.
+const BAND_WEIGHTS_JSON: &str = concat!(
+    env!("CARGO_MANIFEST_DIR"),
+    "/../../priv/algo/band_weights.json"
+);
+
 fn ensure_json_env() {
     ENSURE_ENV.call_once(|| {
-        std::env::set_var(
-            "PROP_BAND_WEIGHTS_JSON",
-            "../../priv/algo/band_weights.json",
-        );
+        std::env::set_var("PROP_BAND_WEIGHTS_JSON", BAND_WEIGHTS_JSON);
     });
 }
 
@@ -111,8 +119,7 @@ fn ensure_json_env() {
 fn json_override_weights_match_file() {
     ensure_json_env();
 
-    let json_path = "../../priv/algo/band_weights.json";
-    let expected = parse_json_override_weights(json_path);
+    let expected = parse_json_override_weights(BAND_WEIGHTS_JSON);
 
     for band in band_config::all_bands() {
         let w = band.weights();