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.

src/native_core/mapper/mappers/mmc5.rs

@@ -530,11 +530,11 @@ impl Mapper for Mmc5 {
                 cursor += 2;
             }
         }
-        if cursor + 1 <= data.len() {
+        if cursor < data.len() {
             self.irq_scanline_counter = data[cursor];
             cursor += 1;
         }
-        if cursor + 1 <= data.len() {
+        if cursor < data.len() {
             self.sprite_8x16 = data[cursor] != 0;
             cursor += 1;
         }