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/api_contract.md

@@ -35,8 +35,13 @@ The main public API is organized around these groups:
   - `CpuBus`
   - `CpuError`
   - `NativeBus`
+  - `Ppu`
+  - `Apu`
+  - `ApuStateTail`
+  - `ChannelOutputs`
 - High-level runtime:
   - `NesRuntime`
+  - `RuntimeError`
 - Host execution and lifecycle:
   - `RuntimeHostLoop`
   - `ClientRuntime`
@@ -46,12 +51,23 @@ The main public API is organized around these groups:
   - `InputProvider`
   - `VideoOutput`
   - `AudioOutput`
+- Null/stub implementations:
+  - `NullInput`
+  - `NullVideo`
+  - `NullAudio`
+- Audio helpers:
+  - `AudioMixer`
+  - `RingBuffer`
 - Timing and pacing:
   - `FrameClock`
   - `FramePacer`
   - `PacingClock`
   - `NoopClock`
   - `VideoMode`
+- Video constants:
+  - `FRAME_WIDTH`
+  - `FRAME_HEIGHT`
+  - `FRAME_RGBA_BYTES`
 - Input helpers:
   - `JoypadButton`
   - `JoypadButtons`
@@ -59,6 +75,8 @@ The main public API is organized around these groups:
   - `JOYPAD_BUTTONS_COUNT`
   - `set_button_pressed`
   - `button_pressed`
+- State versioning:
+  - `SAVE_STATE_VERSION`
 
 ## Supported Client Flow