RFC: Method-level capability detection for protocol extension

Source PR: #420 · docs/rfcs/0027-capability-detection.md · @ryanleecode

Description

system.feature_supported answers a Method { id } query, so a product can check whether the host has a handler for a wire discriminant. Unregistered discriminants get an UNSUPPORTED_METHOD reply instead of being dropped.

Motivation

Products update continuously while native hosts update on app-release cycles, so new products routinely run against old hosts. An unknown wire ID is dropped silently today and the pending call hangs forever.

Requirements

• HostFeatureSupportedRequest::Method { id }; older hosts fail to decode it and return MalformedFrame, which is the age signal.
• supported is computed from the dispatcher's own wire table, never a static list.
• Dispatch fall-through replies UNSUPPORTED_METHOD echoing request_id; the client rejects with CallError::Unsupported.
• Codegen exposes a supportsMethod("<method>") helper.

Tasks

• RFC document body
• Implementation — rust-core
• Implementation — hosts
  • dotli
  • Desktop
  • iOS
  • Android
  • host-cli

Note: RFC number 0027 is also claimed by #468.