R2 Infra

Part of #591. Membership is carried by the R1-R2 tech debt label and by this issue's
sub-issues. No requirement was absorbed here.

Scope

What R1 left behind, in four strands:

• Wire format and protocol — the flat discriminant's id exhaustion, versioned payload
  wrappers, request cancellation, capability detection, and the navigation and reload verbs.
• Rust core integration — completing the migration onto the shared core, the native
  bindings, and the runtime's module and locking structure.
• Correctness and ops — persisted-state decoding, the renewal ledger, dependency
  resolution for external consumers, and network presets.
• Test infrastructure — a mock host that can be driven without a device, a browser
  end-to-end path, and the CI that exercises them.