chore: fix clippy warnings and update docs to match public API

Fix 9 clippy warnings across mmc5 mapper and desktop frontend.
Sync api_contract.md and architecture.md with actual public surface.

docs/architecture.md

@@ -41,6 +41,10 @@ The workspace is split into four layers:
 - `src/runtime/audio.rs`: interim PCM synthesis from core state
 - `src/runtime/timing.rs`: frame pacing types and video timing
 - `src/runtime/types.rs`: public joypad-related types and helpers
+- `src/runtime/constants.rs`: frame dimensions and video constants
+- `src/runtime/error.rs`: `RuntimeError` type definitions
+- `src/runtime/ring_buffer.rs`: lock-free ring buffer for audio sample transport
+- `src/runtime/adapters.rs`: adapter bridge types (behind `adapter-api` feature)
 - `src/runtime/host/io.rs`: host IO traits and null implementations
 - `src/runtime/host/executor.rs`: per-frame execution unit
 - `src/runtime/host/clock.rs`: clock abstraction and pacing implementations