[johngrimmreaper]

Mega CD currently defaults to the three-button Control Pad even though the six-button Fighting Pad is generally the better default for games that support it.

This change makes Fighting Pad the default controller for Mega CD.

Existing game-specific compatibility behavior is preserved, including the special Controller Port 2 handling for Corpse Killer.

Known six-button-incompatible games are handled separately by the compatibility override work, so they can still fall back to the three-button Control Pad when required.

Validation:

• Ubuntu 24.04 Noble sbuild completed successfully
• Tested together with controller persistence and compatibility override changes

[johngrimmreaper]

Related to #1644.

I split the controller work into separate PRs so each behavior can be reviewed independently:

• desktop-ui: persist controller selection #2657 — persist controller selection (merged)
• md: default to Fighting Pad #2660 — default Mega Drive to Fighting Pad
• mcd: default to Fighting Pad #2663 — default Mega CD to Fighting Pad
• md: force 3-button mode for incompatible games #2659 — force 3-button mode for incompatible Mega Drive games
• mcd: force 3-button mode for incompatible games #2658 — force 3-button mode for incompatible Mega CD games

Together, these changes cover the persistent controller-selection and six-button-by-default parts of #1644 while preserving compatibility with games that require a three-button controller.