CI: fail when an example's trace exceeds its size budget #496

Part of #494.

Problem

Nothing stops a change from making traces heavier, and two planned features (#494's page-resources and dense filmstrip work) grow them by design. Today a size regression is only noticed when someone opens a zip.

Proposal

The examples-smoke CI job already records real trace zips for all four adapters and checks them with pnpm smoke:assert against packages/conformance/smoke/expectations/*.json. Add a byte budget per expectation file and fail when an example's zips exceed it, naming the example, the size and the budget.

Budgets should start from the sizes the job currently produces, with headroom, and be lowered when a size win lands so the win cannot be undone silently.