docs(testing): coverage guide should not prescribe a removed uploader (#753)

Completes the Codecov removal in this repo. `wiki/testing/guide.md` had
a copy-paste block telling contributors to add `uses:
codecov/codecov-action@v3` with a `coverage.lcov` file — Codecov is not
installed here or on the account, so following the guide would produce
the same red CI this sweep removed, and Dependabot would eventually
re-pin the tag.

Rewritten to state what `.github/workflows/ci.yml` really does
(visibility-only coverage via `bisect-ppx-report` +
`actions/upload-artifact`) and to point at
`docs/standards/TESTING.adoc`, so the doc and the CI can't drift again.
No functional workflow change: this file is documentation only.

Author: hyperpolymath

wiki/testing/guide.md

@@ -609,15 +609,21 @@ aspm coverage --format lcov --output coverage.lcov
 
 ### Coverage in CI
 
+Coverage is visibility-only: the report is attached to the run as an artifact, not shipped
+to a third-party service. This is what `.github/workflows/ci.yml` actually does (see
+`docs/standards/TESTING.adoc`, "Coverage (visibility-only)") — no upload token, no external
+account, and nothing for Dependabot to keep bumping.
+
 ```yaml
 # .github/workflows/ci.yml
 - name: Run tests with coverage
   run: aspm test --coverage
 
-- name: Upload coverage
-  uses: codecov/codecov-action@v3
+- name: Upload coverage report
+  uses: actions/upload-artifact@v7.0.1
   with:
-    file: coverage.lcov
+    name: coverage-lcov
+    path: coverage.lcov
 ```
 
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