Implement Peripheral Slot Adapter for External Plugins

[maxolasersquad]

Objective

Bridge slot peripheral plugins implementing the C99 Peripheral_t vtable into LinApple's internal slot management system (Slots 1–7).

Scope & Requirements

• Adapt the plugin Peripheral_t C99 vtable into LinApple's slot interface.
• Route hardware memory operations:
  • I/O space read/write ($C0n0–$C0nF for Slot n)
  • Slot PROM read/write ($Cn00–$CnFF)
  • Expansion ROM mapping ($C800–$CFFF)
  • Frame updates and periodic think ticks
• Connect IRQ and NMI line assertion from the plugin back to the emulated 6502 bus.
• Provide procedural slot management functions:
  • plugin_slot_insert(SlotManager* slot_mgr, int slot_index, Peripheral_t* peripheral) -> bool
  • plugin_slot_eject(SlotManager* slot_mgr, int slot_index) -> void
• Ensure clean destruction of card state upon ejection or machine reset.

Testing & Verification

• Insert a mock peripheral plugin into Slot 4 and verify CPU reads and writes to $C0C0–$C0CF reach the plugin's I/O handlers.
• Verify Slot PROM reads at $C400–$C4FF map to the card's firmware PROM buffer.
• Verify Expansion ROM reads at $C800–$CFFF route to the active card when selected.
• Assert an IRQ from the mock plugin and verify the 6502 CPU registers the pending interrupt line, and verify deasserting IRQ clears it.
• Verify periodic frame update ticks are dispatched to the slotted card's update / think function.
• Verify inserting a card into an invalid slot index (< 1 or > 7) or passing null pointers returns false and fails gracefully.
• Eject the plugin from Slot 4 and verify memory accesses to Slot 4 revert to floating bus / empty slot behavior.
• Verify inserting a new card into an occupied slot cleanly unloads the previous card before binding the new one.

Acceptance Criteria

• CPU reads and writes to assigned slot memory spaces invoke the plugin card handlers.
• Plugin cards can trigger and release 6502 IRQ interrupts.
• Ejecting or replacing a plugin slot card cleanly releases resources without bus corruption.