Run the TanStack Start e2e suite on Bun, Deno and Cloudflare from one app

[JPeer264]

Run the TanStack Start e2e suite on Node, Bun, Deno and Cloudflare from one app, following #24598 (react-router-8-framework). Node stays the required job. Bun, Deno and Cloudflare start as optional variants and become required once green.

Depends on #24598 (runtime matrix key, runtimeEntryPlugin, README convention) and #24612 (Bun diagnostics channels, missing http.server span).

Apps

• Base: tanstackstart-react
• Fold in, then delete: tanstackstart-react-cloudflare
• New variants: bun, deno

Cover on every runtime

The existing suite runs unchanged and branches on RUNTIME only where runtimes are expected to differ. Add assertions where the suite has none today:

• Errors
• Parameterized route names
• Per-request isolation
• Incoming trace continuation
• Outgoing fetch with sentry-trace and baggage
• Logs

Findings to confirm (runtime audit, September 2026, develop 6f2da7483d; database spans were measured with mysql or mysql2):

• Bun: route names: broken; isolation: broken; incoming trace: broken; outgoing fetch: broken; db spans: broken. test(bun): Run the auto-instrumentation suites with bundled scenarios #24612 targets the missing request span on Bun.
• Deno: route names: broken; outgoing fetch: broken. deno: Outgoing fetch gets no span and no trace propagation #23891 has added outgoing fetch spans to @sentry/deno since.
• Cloudflare: everything measured worked.

Setups the audit ran: Bun, Deno, Cloudflare

Done when

• tanstackstart-react runs its suite on Node, Bun, Deno and Cloudflare
• Every variant is green and required, or this issue records why a runtime is not supported
• tanstackstart-react-cloudflare is deleted
• SDK defects the variants surface are fixed here, or split into their own issue when the fix is large or shared with other libraries

[linear-code]

JS-3840