ci: verify the client SDK on Deno and Bun

## Goal Add native compatibility checks for `@streamweld/client` on Deno and Bun. The package intentionally uses no Node-only APIs, but today CI verifies it only through Node/Vitest. ## Scope - Add pinned Deno and Bun setup steps or jobs to `.github/workflows/ci.yml`. - Add a small smoke test that imports the built ESM package and consumes a deterministic SSE response. - Keep the existing Node/Vitest matrix unchanged. - Document the tested runtime versions in `packages/client/README.md`. ## Acceptance Criteria - [ ] The smoke test runs under both Deno and Bun. - [ ] CI fails if the SDK imports a Node-only API. - [ ] Runtime versions are pinned for reproducibility. - [ ] `pnpm typecheck`, `pnpm test`, and `pnpm build` remain green. ## Contributor Notes The existing deterministic fixtures in `packages/client/src/client.test.ts` show the expected SSE event shapes. Please keep the new test focused on runtime compatibility rather than duplicating the full Vitest suite.