Add hotswappable CD-ROM mode with runtime disc switching

[GenSayer]

Introduces a hotswappable flag for CD-ROM devices that enables on-the-fly disc switching via keyboard shortcuts (RCtrl+F12 native, Ctrl+F12 GUI) without pre-configuring a changer queue or restarting the emulator.

New features:

• hotswappable = true per-drive config (iris.toml + GUI checkbox)
• Runtime disc loading via file picker (RCtrl+F12 / Ctrl+F12)
• Empty tray on startup when path omitted (hotswappable only)
• GUI config editor: browse button loads discs immediately
• GUI: "Extra changer discs" hidden when hotswappable=true

Validation:

• Errors when hotswappable=false + empty path (suggests enabling, cleaner error message than before)
• Errors when hotswappable=true + discs list (mutually exclusive)
• Ctrl+F12 disabled with helpful message when hotswappable=false

Console logging:

• "CD-ROM hotswap: ejected '', tray empty" (hotswappable eject)
• "CD-ROM changer: switched to ''" (legacy changer cycle)
• "SCSI SGI_EJECT: tray emptied" (hotswappable SGI eject)

Config example:
[scsi.4] cdrom = true hotswappable = true # path optional — start with empty tray

Changes: 13 files, 699 lines

• Core: config.rs, scsi.rs, wd33c93a.rs, machine.rs, main.rs, ui.rs
• GUI: config_ui.rs, main.rs, handle.rs, scsi_menu.rs, dialogs/new_machine.rs
• Docs: iris.toml
• Build: Cargo.toml (added rfd for file picker)