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

Add an additive coverage job to ci.yml that mirrors the test job's
sibling-checkout layout and pinned 1.97.0 toolchain, runs cargo llvm-cov,
and uploads to Codecov tokenlessly (fail_ci_if_error:false). Nothing in
the release cascade depends on it. Also add codecov.yml (informational,
no PR comment) and a README with the coverage badge.

codecov.yml

@@ -0,0 +1,19 @@
+# Coverage is informational only: it never blocks a merge or a release. The
+# release cascade is gated by ci.yml / qa.yml and the local precommit, not by
+# a coverage threshold.
+coverage:
+  status:
+    project:
+      default:
+        informational: true
+    patch:
+      default:
+        informational: true
+
+# No PR comment noise; the badge and the Codecov UI are enough.
+comment: false
+
+# Exclude non-product code from the coverage denominator.
+ignore:
+  - "tests/**"
+  - "ci/**"