Implement concrete iOS runtime companion client for mobile parity

[yacosta738]

Goal

Implement the concrete iOS runtime companion client required to move the mobile iOS path from fail-closed/provider-only wiring to a real runtime-backed transport.

Context

Follow-up from DALLAY-179. The mobile parity change now has an explicit IosRuntimeCompanionClientProvider boundary and registration hooks, but there is still no concrete provider/client implementation in the repo.

Scope

• Define the approved iOS companion transport/client implementation
• Implement the concrete IosRuntimeCompanionClient and provider wiring
• Integrate it with the existing iOS runtime dependency path
• Add targeted validation for the concrete iOS runtime path

Acceptance Criteria

• iOS no longer relies on MissingIosRuntimeCompanionClientProvider in active runtime wiring
• A concrete IosRuntimeCompanionClient implementation exists and can be installed/used by the mobile app
• iOS runtime path reaches real runtime-backed behavior instead of fail-closed fallback
• Verification evidence exists for the concrete iOS runtime transport path

[linear]

DALLAY-213