ci: add non-required Codecov coverage job, config, and badge

Add an additive coverage job to CI that generates lcov via cargo llvm-cov
and uploads to Codecov (tokenless). Mirrors the test job's pinned 1.94
toolchain and dual freemkv-unlock checkout for the path dep. Informational
only: no gate depends on it. Adds codecov.yml and a README badge.

codecov.yml

@@ -0,0 +1,18 @@
+# Coverage is informational only: it never fails CI or blocks a merge/release.
+# The `coverage` job in .github/workflows/ci.yml is non-required and no gate
+# depends on it.
+coverage:
+  status:
+    project:
+      default:
+        informational: true
+    patch:
+      default:
+        informational: true
+
+comment: false
+
+ignore:
+  - "tests"
+  - "examples"
+  - "benches"