Replace absolute boot gzip ceiling with PR delta gate #4464

Problem

Cloud deploys fail when boot-set gzip grows by a few bytes over the pinned `totalGzipBytes` ceiling in `boot-size-budget.json`. Release v1.3.160 needed an emergency ceiling bump (#4463) instead of shipping product fixes.

Approved direction (Tyler)

1. Deploy/release path: keep measuring boot gzip when building the web image, log the number, and never fail deploy on absolute bundle size.
2. PR CI: build merge base and PR head (deploy-shaped config), compute gzip delta, post/update one sticky PR comment (e.g. `Boot bundle: 519.8 KB gzip (+0.4 KB vs main)`).
3. Gate: fail the PR check only when delta exceeds +5 KB gzip in one PR. Allow override with a `bundle-size-ok` label for intentional growth.
4. Budget file/docs: simplify `boot-size-budget.json` (drop enforced absolute ceiling; keep informational baseline) and update performance/CI docs.

Acceptance

• Staging/production web image builds do not exit non-zero on absolute gzip ceiling.
• Required PR check reports delta vs `main` and enforces +5 KB unless labeled.
• Docs describe the new model.