Add PR check reporting npm package size changes

[timotheeguerin]

Summary

Adds a PR check that reports how a change affects the published npm package size of the workspace libraries and posts the result as a sticky comment. Purely informational — it never fails the check.

Since these TypeSpec libraries are not bundled (they build with tsc and publish dist/**), "size" here is the npm tarball that npm pack produces — both packed (gzipped .tgz) and unpacked sizes.

How it works

Two workflows, following the secure two-stage pattern already used by core/.github/workflows/commenter.yml (so it works for fork PRs without exposing a write token to untrusted code):

• package-size.yml (pull_request, read-only, scoped via paths:) — builds + measures the PR head, checks out the base commit, builds + measures it, diffs the two, and uploads the rendered comment as an artifact.
• package-size-comment.yml (workflow_run, pull-requests: write) — downloads the artifact and posts/updates a single sticky comment (keyed by a hidden marker).

New scripts under eng/scripts/bundle-size/ (run with tsx):

• measure.ts — enumerates publishable packages via listPackages() (skips private/unnamed), runs npm pack --dry-run --json --ignore-scripts per package.
• compare.ts — renders a markdown table of packed/unpacked deltas (%), flags 🆕 added / 🗑️ removed packages, and collapses unchanged packages into a <details> block.
• utils.ts — byte/delta/percent formatting helpers.

Example comment

📦 Package size report

1 package changed size compared to the base branch.

Package	Packed (base → head)	Δ Packed	Unpacked (base → head)	Δ Unpacked
`@azure-tools/typespec-azure-core`	126.32 KB → 131.52 KB	+5.20 KB (+4.1%)	685.38 KB → 697.10 KB	+11.72 KB (+1.7%)

Notes

• Measures all publishable workspace packages (includes core/packages/*); in a normal Azure PR those stay in the collapsed "unchanged" section.
• Cost: two full builds per qualifying PR — mitigated by the paths: filter and a dedicated non-required workflow.

Validation

• Scripts run locally (measured 51 packages; test fixtures skipped gracefully); simulated grow/shrink/add/remove diff renders correctly.
• tsc -p tsconfig.eng.json passes, prettier clean, actionlint passes on both workflows, chronus verify confirms no changelog entry is required (eng/CI-only change).

[azure-sdk-automation]

No changes needing a change description found.

[azure-sdk-automation]

You can try these changes here

🛝 Playground	🌐 Website