ci: add non-required Codecov coverage job

Add an additive, non-required coverage job to CI that runs cargo
llvm-cov and uploads lcov to Codecov (tokenless, fail_ci_if_error
false). Mirrors the test job's environment and pinned 1.90.0 toolchain;
--locked is omitted to match this crate's gitignored-lockfile policy.
Adds codecov.yml (informational, comment off) and a README badge.

codecov.yml

@@ -0,0 +1,16 @@
+coverage:
+  status:
+    project:
+      default:
+        informational: true
+    patch:
+      default:
+        informational: true
+
+comment: false
+
+ignore:
+  - "tests"
+  - "**/tests"
+  - "benches"
+  - "examples"