ci(codecov): webpack bundle-stats job

[ng]

Summary

• Next.js 16 defaults to Turbopack, which skips the webpack() hook in next.config.mjs, so @codecov/nextjs-webpack-plugin never runs during the deploy build (verified on commit fdce534 — no Codecov upload in Build & Package logs).
• Adds bundle-stats.yml: runs next build --webpack as a parallel CI job purely so the Codecov plugin can register and upload bundle stats via OIDC. The deploy build in build.yml stays on Turbopack.

Caveat

Reports webpack bundle sizes, not the Turbopack bundles we actually ship. Reasonable proxy for regression detection; not exact for absolute size. Revisit when Codecov ships a Turbopack plugin.

Test plan

• Workflow runs on this PR and uploads bundle analysis for sleepypod-core to Codecov.
• Build & Package (Turbopack) remains untouched.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 8447df3a-e538-4adb-8a51-687f89d66338

📥 Commits

Reviewing files that changed from the base of the PR and between cb389c9 and d9fcb9d.

📒 Files selected for processing (3)

• .github/workflows/build.yml
• .github/workflows/bundle-stats.yml
• next.config.mjs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/codecov-bundle-stats

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

📢 Thoughts on this report? Let us know!

[ng]

Closing — next build --webpack as a CI-only side-build for Codecov bundle stats isn't a viable path on Next.js 16.

Why:

• Turbopack is the default in v16; the webpack flag is a sunset escape hatch, not a maintained build path. Dep authors target Turbopack, so webpack-mode hits bit-rot on every upgrade (we already hit two unrelated ones in one build: @trpc/react-query v11's tsdown ESM interop, and Babel's regen-unicode-properties choking on @messageformat/parser's \p{Pattern_Syntax}).
• The measured artifact (webpack) isn't what ships (Turbopack) — different tree-shaking/splitting/minification means trend lines diverge from the real bundle. False-positive blocks or missed real regressions.
• Codecov has no Turbopack plugin today. Until they ship one, there's no clean path.

What stays: the OIDC switch in #538 is harmless — the webpack() hook is dead code under Turbopack but costs nothing and lights up automatically the day Codecov ships Turbopack support.

If we ever want bundle-regression visibility before then: diff .next/ route chunk sizes from the actual Turbopack build and post a PR comment. ~30 lines of CI, no vendor, no second bundler.