fix(rtg): align P96 driver contract

[midwan]

Summary

• move ZZ9000 RTG per-board state out of file globals and into BoardInfo->CardData
• switch the runtime P96 identity to native BT_MNT_ZZ9000 with matching packaged Picasso96Settings
• preserve the working ZZ9000 hardware sprite behavior while returning the expected P96 BOOL result
• add P96 contract cleanups for memory-space fields, callback prototypes, split handling, mode flags, and $VER: formatting
• add installer migration messaging and backups for existing Devs:Picasso96Settings

Why

P96 expects drivers to support multiple identical boards without shared instance state, and the native ZZ9000 BoardType needs a settings migration path for upgrades from the legacy uaegfx identity.

Validation

• built rtg/ZZ9000.card with the Docker m68k Amiga compiler
• ran git diff --check
• tested ZZ9000.card 2.4 on hardware; mouse pointer is visible and no problems reported

Upgrade Notes

The installer backs up an existing settings file to Devs:Picasso96Settings.pre-ZZ9000-2.4 before installing the migrated native ZZ9000 settings file.