feat(imaging): add pure-Rust Skia backend (#112)

ci/ci-coverage.json

@@ -13,6 +13,7 @@
     "minimal-features",
     "jpeg2000",
     "rust-j2k",
+    "rust-skia",
     "skia-binary",
     "program-offline-smoke",
     "clippy",
@@ -104,8 +105,8 @@
       "workflow": "skia.yml",
       "responsibility": "Default and Skia feature tests plus package listing",
       "destination": "required",
-      "checks": ["skia-binary", "workspace-tests"],
-      "rationale": "The required gate proves the exact ARM64 GNU archive is installed and forbids source compilation."
+      "checks": ["rust-skia", "skia-binary", "workspace-tests"],
+      "rationale": "A dedicated dependency-free job tests the Rust-only codec graph, while the required gate proves the exact ARM64 GNU archive is installed and forbids source compilation."
     },
     {
       "workflow": "supply-chain.yml",