Bundle-size CI gate for widget package

[PAMulligan]

Goal

Protect end-user page weight by failing CI if the widget bundle grows past a defined budget.

Acceptance criteria

• Add size-limit config with budgets for ESM + CJS builds
• Workflow runs on PRs and posts a comment with delta vs main
• Fail PR when delta exceeds budget without an accepted: bundle increase label
• Document budget rationale in CONTRIBUTING

Milestone

v1.2.0