chore(ci): add codecov config to reduce noise on small refactors (#1044)

The repo currently has no `codecov.yml`, so Codecov uses its strict
defaults: any coverage drop (project) and any patch under 100% are
reported as failing statuses on PRs.
This produces frequent red checks on small refactors even when coverage
is unchanged or trivially impacted (e.g. #1035, #1038).

This change adds a minimal `codecov.yml` that keeps useful signal while
removing the noise:

- `coverage.status.project.default.threshold: 1%` — allow up to a 1%
project-coverage drop without failing.
- `coverage.status.project.default.if_ci_failed: success` — don't
double-report when the CI run itself failed.
- `coverage.status.patch.default.informational: true` — still report
patch coverage, but never block PRs on it.
- `comment.require_changes: true` — only post a PR comment when coverage
actually changes.

Validated with `curl --data-binary @codecov.yml
https://codecov.io/validate` → `Valid!`.

Real coverage regressions will still surface (anything > 1% project
drop), and contributors keep getting patch-coverage feedback — it just
won't block.

Author: Tranquility2

codecov.yml

@@ -0,0 +1,26 @@
+# https://docs.codecov.com/docs/codecov-yaml
+coverage:
+  status:
+    project:
+      default:
+        target: auto
+        # Allow small coverage fluctuations without failing the check.
+        threshold: 1%
+        # Don't fail codecov if the CI run itself failed (avoids double-noise).
+        if_ci_failed: success
+    patch:
+      default:
+        # Report patch coverage, but never block the PR on it.
+        informational: true
+
+# Recompute the base when code is removed so refactors don't appear to
+# regress overall coverage.
+codecov:
+  notify:
+    after_n_builds: 1
+  require_ci_to_pass: true
+
+comment:
+  layout: "reach,diff,flags,files"
+  behavior: default
+  require_changes: true  # only post a PR comment when coverage actually changed