Document MMCE+MX4SIO limitation

Author: NathanNeurotic

.github/workflows/cl.yml

@@ -291,6 +291,7 @@ jobs:
             Test Sheet: https://github.com/NathanNeurotic/PlayStation2-Basic-BootLoader-Extended/blob/main/BETA_TEST_CHECKLIST.md  
 
             ### PS2BBLE.zip contains all versions
+            Note: MMCE + MX4SIO combined variants are intentionally omitted (unsupported in embedded IRX mode).
           prerelease: true
           files: release-artifacts/PS2BBLE.zip
         env:

README.md

@@ -464,6 +464,7 @@ make
 ```
 
 cl commonly builds multiple variants by enumerating feature flags (embedded vs runtime drivers, device support, chainload modes, etc.).
+Compatibility note: "normal" builds embed IRX for selected devices, and MMCE + MX4SIO embedded modes are mutually exclusive. Automated prerelease artifacts omit MMCE+MX4SIO combinations accordingly.
 
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