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)

[techomancer]

lets skip this "hotswappable" flag
if it is cdrom with no file defined at start so be it.
then you can load an iso and it will be available
if there is only one iso and it is ejected then it becomes empty
but you can always queue more isos, so if there is more than one, it should cycle between them
i think this will most reasonable behavior

[GenSayer]

CD swapping logic changed to the behavior you wanted. Is the code good enough to be merged now?

[techomancer]

i think it looks but lets squash it into single commit the churn doesnt bring any value to the history