[#877] Bus: Set IO_MEM_EXECUTABLE_IO for all non-IMapped peripherals

Previously, only ArrayMemory peripherals had the IO_MEM_EXECUTABLE_IO
flag set, allowing instruction fetch via I/O callbacks. Any other
non-IMapped peripheral (e.g. dynamically compiled C# peripherals)
would trigger cpu_abort when the CPU tried to execute code from their
address range.

Generalize the check from `peripheral is ArrayMemory` to
`!(peripheral is IMapped)` in three places:
- SystemBus.RegisterInner: set flag when peripheral is first registered
- SystemBus.MoveRegistrationWithinContext: preserve flag on re-registration
- Machine.PostCreationActions: set flag for peripherals registered before CPU

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: neilberkman

src/Emulator/Main/Core/Machine.cs

@@ -1027,10 +1027,11 @@ public void PostCreationActions()
                 }
             }
 
-            // Register io_executable flags for all ArrayMemory peripherals
+            // Register io_executable flags for all non-IMapped peripherals so that
+            // instruction fetches from them go through I/O callbacks instead of aborting.
             foreach(var context in SystemBus.GetAllContextKeys())
             {
-                foreach(var registration in SystemBus.GetRegistrationsForPeripheralType<Peripherals.Memory.ArrayMemory>(context))
+                foreach(var registration in SystemBus.GetRegistrationsForPeripheralType<IBusPeripheral>(context).Where(r => !(r.Peripheral is IMapped)))
                 {
                     var range = registration.RegistrationPoint.Range;
                     var perCore = registration.RegistrationPoint.Initiator;

src/Emulator/Main/Peripherals/Bus/SystemBus.cs

@@ -1260,7 +1260,7 @@ public void MoveRegistrationWithinContext(IBusPeripheral peripheral, BusRangeReg
                 AddMappingsForPeripheral(peripheral, newRegistration, context);
             }
 
-            if(peripheral is ArrayMemory)
+            if(!(peripheral is IMapped))
             {
                 foreach(var cpu in GetCPUsForContext<ICPUWithMappedMemory>(context))
                 {
@@ -2054,6 +2054,16 @@ private void RegisterInner(IBusPeripheral peripheral, PeripheralAccessMethods me
                 });
                 // let's add new mappings
                 AddMappingsForPeripheral(peripheral, registrationPoint, context);
+                // For non-IMapped peripherals, mark the range as executable I/O so that
+                // instruction fetches go through I/O callbacks instead of causing cpu_abort.
+                if(!(peripheral is IMapped))
+                {
+                    foreach(var cpu in GetCPUsForContext<ICPUWithMappedMemory>(context))
+                    {
+                        var range = registrationPoint.Range;
+                        cpu.RegisterAccessFlags(range.StartAddress, range.Size, isIoMemory: true);
+                    }
+                }
                 // After adding new mappings, if the address range is locked, the mappings possibly have to be modified/unmapped on the CPU's side
                 // RelockRange to make this happen
                 if(IsAddressRangeLocked(registrationPoint.Range, context))