ci(codecov): upload unit-suite lcov on every PR + README badge

Per-PR coverage signal via Codecov. Unit suite only — `yarn coverage` for
the full merged (unit + e2e) report stays a follow-up job (30+ min is too
expensive per PR; issue #267 plans a nightly or on-label job for that).

- Adds `yarn coverage:unit` step to build_artifact (Linux, Node 22) after
  lint/build, then uploads coverage/lcov.info via codecov-action@v5.
  build_artifact already runs on every PR and produces the distributable
  bundle, so coverage fits naturally without adding a separate job.
- codecov.yml: informational status (never fail a PR on coverage swings,
  since e2e coverage isn't uploaded yet), ignore compiled output / tests
  / docs-site / dictionary, carryforward disabled on the `unit` flag.
- README: Codecov badge next to CI badge, labeled "Coverage (unit)" so
  the scope is obvious at a glance.

Requires: Codecov GitHub App installed on yao-pkg and CODECOV_TOKEN set
as a repo secret (optional on public repos but recommended for PR uploads).

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Author: robertsLando

.github/workflows/ci.yml

@@ -57,6 +57,21 @@ jobs:
             prelude/sea-bootstrap.bundle.js
           retention-days: 1
 
+      # Coverage upload — unit-suite only. The full `yarn coverage` (unit + e2e)
+      # is 30+ min and not worth paying on every PR; a follow-up can add a
+      # nightly or labeled job for the merged report. The `build` matrix runs
+      # `yarn test:unit` on 3 OS × Node 22/24 to verify platform behavior; this
+      # is the single canonical upload for Codecov.
+      - if: needs.changes.outputs.code == 'true'
+        run: yarn coverage:unit
+      - if: needs.changes.outputs.code == 'true'
+        uses: codecov/codecov-action@v5
+        with:
+          files: coverage/lcov.info
+          flags: unit
+          fail_ci_if_error: false
+          token: ${{ secrets.CODECOV_TOKEN }}
+
   # Sanity check: `yarn build` succeeds on every supported OS + Node combo.
   # Runs in parallel with tests (they don't consume this job's output) so
   # failures still mark the PR red without gating the test jobs.

README.md

@@ -10,6 +10,7 @@
 
 <p align="center">
   <a href="https://github.com/yao-pkg/pkg/actions/workflows/ci.yml"><img src="https://github.com/yao-pkg/pkg/actions/workflows/ci.yml/badge.svg" alt="Build Status" /></a>
+  <a href="https://codecov.io/gh/yao-pkg/pkg"><img src="https://codecov.io/gh/yao-pkg/pkg/branch/main/graph/badge.svg" alt="Coverage (unit)" /></a>
   <a href="https://www.npmjs.com/package/@yao-pkg/pkg"><img src="https://img.shields.io/npm/v/@yao-pkg/pkg" alt="npm version" /></a>
   <a href="https://www.npmjs.com/package/@yao-pkg/pkg"><img src="https://img.shields.io/npm/dm/@yao-pkg/pkg" alt="npm downloads" /></a>
   <a href="https://github.com/yao-pkg/pkg/blob/main/LICENSE"><img src="https://img.shields.io/npm/l/@yao-pkg/pkg" alt="license" /></a>

codecov.yml

@@ -0,0 +1,38 @@
+# Codecov config — keep coverage informational, don't gate PRs on it.
+# Upload is unit-suite only today; nightly / labeled merged uploads may land
+# as a follow-up (see #267 Phase 2 acceptance criteria).
+coverage:
+  status:
+    project:
+      default:
+        # Report but never fail the PR. Coverage swings with e2e-vs-unit
+        # scope changes, and the e2e suite isn't on Codecov yet.
+        target: auto
+        threshold: 5%
+        informational: true
+    patch:
+      default:
+        # New lines should be covered by the unit suite when reasonable, but
+        # many changes land in integration-heavy modules (walker, producer,
+        # packer) that stay e2e-tested. Warn, don't fail.
+        informational: true
+
+comment:
+  layout: "reach, diff, files"
+  behavior: default
+  require_changes: true
+
+ignore:
+  - "lib-es5/**"
+  - "test/**"
+  - "prelude/**"
+  - "dictionary/**"
+  - "scripts/**"
+  - "docs-site/**"
+  - "**/*.d.ts"
+
+flags:
+  unit:
+    paths:
+      - lib/
+    carryforward: false