Track remaining work for the shared Rust core integration on iOS

[pgherveou]

Context

Track the remaining work to land the initial Rust integration , complete the migration to the shared Rust core, and address the main production-readiness gaps.

Initial integration blockers

• Permission-gate outbound network requests and WebRTC access inside the container.
• Complete Chat modality support (#326, specification).
• Add Extrinsic V5 support. Related WIP PRs: frame-decode #104, Subxt #2268, and TrUAPI #333.

Full integration follow-up

• Move SSO to the shared Rust core. Related work: polkadot-app-ios-v2#1313.
• Implement RFC 0017 in Rust.
• Route payment functions and subscriptions for products such as T3rminal through the native/shared core, or add temporary callbacks for payment subscriptions.
• Add background processing for statement-store allowance renewal (#308) and Coinage.

Performance and resilience

• Investigate the PAPI provider memory issue when the app is backgrounded. @ERussel to provide details.
• Support background execution for in-flight async work such as extrinsic submission, using a Rust callback/task abstraction. @ERussel to provide details.
• Revalidate OS permissions before each protected operation because the host or OS may suspend, revoke, or reset them.
• Add diagnostics that notify the user when an operation takes too long.