plate-test: app-neutral print color scenes, checked by PDF structure and Ghostscript plates

craft-plate-test writes JSON scenes through craft-pdf and checks the written PDF's
structure (CI, under a second) and, locally only, its separated plates through
Ghostscript tiffsep (scripts/plate-check.sh). Apps can run the same scenes through
their own export as a dev-dependency.

.github/workflows/pdf.yml

@@ -3,9 +3,9 @@ name: Shared PDF
 on:
   push:
     branches: [main]
-    paths: ["crates/pdf/**", "vendor/hayro*/**", "vendor/krilla/**", "scripts/test-pdf-backends.sh", "scripts/*fetch-pdf-test-fonts.py", "Cargo.toml", "Cargo.lock", ".github/workflows/pdf.yml"]
+    paths: ["crates/pdf/**", "crates/plate-test/**", "vendor/hayro*/**", "vendor/krilla/**", "scripts/test-pdf-backends.sh", "scripts/*fetch-pdf-test-fonts.py", "Cargo.toml", "Cargo.lock", ".github/workflows/pdf.yml"]
   pull_request:
-    paths: ["crates/pdf/**", "vendor/hayro*/**", "vendor/krilla/**", "scripts/test-pdf-backends.sh", "scripts/*fetch-pdf-test-fonts.py", "Cargo.toml", "Cargo.lock", ".github/workflows/pdf.yml"]
+    paths: ["crates/pdf/**", "crates/plate-test/**", "vendor/hayro*/**", "vendor/krilla/**", "scripts/test-pdf-backends.sh", "scripts/*fetch-pdf-test-fonts.py", "Cargo.toml", "Cargo.lock", ".github/workflows/pdf.yml"]
   workflow_dispatch:
 
 permissions:
@@ -86,3 +86,13 @@ jobs:
           cargo install wasm-bindgen-cli --version "$runner_version" --locked
       - run: cargo test -p craft-pdf --all-features --locked --target wasm32-unknown-unknown
       - run: cargo test --manifest-path crates/pdf/read-only-contract/Cargo.toml --locked --target wasm32-unknown-unknown
+  plate-test:
+    name: Plate test scenes
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: Swatinem/rust-cache@v2
+        with:
+          key: craft-plate-test
+      - run: cargo test -p craft-plate-test --locked