Remove legacy JVM-specific file system, shell, and related implementations; migrate to platform-agnostic and common main modules.

src/commonMain/kotlin/mtmc/emulator/MTMCIO.kt

@@ -0,0 +1,26 @@
+package mtmc.emulator
+
+class MTMCIO {
+  var value: Int = 0
+
+  internal enum class Buttons(val mask: Int) {
+    UP(128),
+    DOWN(64),
+    LEFT(32),
+    RIGHT(16),
+    START(8),
+    SELECT(4),
+    B(2),
+    A(1)
+  }
+
+  fun keyPressed(key: String) {
+    val button = Buttons.valueOf(key.uppercase())
+    value = value or button.mask
+  }
+
+  fun keyReleased(key: String) {
+    val button = Buttons.valueOf(key.uppercase())
+    value = value and button.mask.inv()
+  }
+}