Add GitHub Checks output mode for bundle analysis

[mohsen1]

Summary

This adds a new outputMode for bundle analysis publishing:

• comment: keep the existing PR comment behavior
• check: publish the full report to a real GitHub Check Run and only post a short PR comment when increases exceed thresholds
• both: publish the full report to both the Check Run and the PR conversation

It also adds threshold-aware managed comment behavior, workflow input overrides, neutral handling for missing base artifacts, and support for buildOutputDirectory resolution from package.json.

Key changes

• add real Check Run publishing in the reusable workflow
• keep default behavior backward-compatible for comment mode
• only post short regression comments in check mode when thresholds are exceeded
• delete threshold comments again when the PR goes back under threshold
• update both mode comments in place
• support workflow input overrides for output mode and thresholds
• resolve custom build output directories from workflow input or package.json
• refactor compare.js and report.js for better testability
• add coverage for missing artifacts, budget math, zero-size pages, App Router-style builds without /_app, and mode-specific behavior
• opt JavaScript actions into Node 24 to avoid Node 20 action deprecation warnings

Validation

Local:

• npm test

Live end-to-end validation in a public Next.js repo:

• Validation repo: https://github.com/mohsen1/nextjs-bundle-analysis-e2e
• check mode regression, then thresholds raised so the managed comment was removed while the custom check remained: Validate bundle increase reporting mohsen1/nextjs-bundle-analysis-e2e#1
• check mode docs-only PR with 1 B thresholds showed the expected noise from tiny global drift, which is why practical thresholds matter: Validate no-bundle-change behavior mohsen1/nextjs-bundle-analysis-e2e#2
• both mode regression with a follow-up push updated the existing managed comment in place: Validate both mode comment updates mohsen1/nextjs-bundle-analysis-e2e#3
• custom buildOutputDirectory from package.json (dist) without workflow input override: Validate custom dist output resolution mohsen1/nextjs-bundle-analysis-e2e#4
• repeated docs-only PR updates in check mode with practical thresholds (1024 bytes) produced No bundle changes checks and no managed PR comments across multiple pushes: Validate docs-only behavior with practical thresholds mohsen1/nextjs-bundle-analysis-e2e#5
• check mode large regression produced the full report in Checks and a short threshold comment in the PR: Validate check-mode regression comment mohsen1/nextjs-bundle-analysis-e2e#6
• comment mode regression produced only the PR comment and no custom Check Run: Validate comment mode regression mohsen1/nextjs-bundle-analysis-e2e#7
• both mode docs-only PR with practical thresholds produced a custom check and no PR comment: Validate quiet both-mode behavior mohsen1/nextjs-bundle-analysis-e2e#8
• comment mode docs-only PR with practical thresholds and skipCommentIfEmpty produced no PR comment: Validate quiet comment-mode behavior mohsen1/nextjs-bundle-analysis-e2e#9
• PR against a base branch with no uploaded artifact produced a neutral custom check titled Base bundle artifact missing and no PR comment: Validate missing-base neutral check mohsen1/nextjs-bundle-analysis-e2e#10

Notes

Very small thresholds such as 1 byte are noisy in practice on real Next.js builds. With practical thresholds like 1024, the expected quiet-PR path behaved cleanly in repeated docs-only runs: developers saw the Check Run and did not see a managed PR comment unless there was a meaningful regression.

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