NFC reader interface (RETRO_ENVIRONMENT_GET_NFC_INTERFACE) with amiibo support

[GavinDarkglider]

What

A core-facing NFC tag reader interface, plus a frontend implementation that can serve tags from a physical reader or from amiibo dumps on disk. Cores that emulate an NFC reader (the 3DS amiibo reader is the motivating case) get real read/write tag access through the frontend instead of each core inventing its own file-drop.

The whole frontend side is opt-in (--enable-nfc, default off). The libretro.h additions are header-only and unconditional, like the other interfaces.

The interface (libretro.h)

RETRO_ENVIRONMENT_GET_NFC_INTERFACE = 99 | RETRO_ENVIRONMENT_EXPERIMENTAL, filling a struct retro_nfc_interface of function pointers (same pattern as LED/rumble/sensor).

• Raw and encryption-agnostic. Tag memory is passed byte-for-byte; amiibo crypto stays in the core.
• Versioned and negotiated. The core writes the version it was built against; the frontend lowers it and fills only those pointers, so the struct can grow without a newer frontend writing past an older core's struct.

Version	Adds
v1	start_scan, stop_scan, get_status, get_tag_info, read, write
v2	set_source (hardware reader vs. software dumps)
v3	get_source_count, get_source_info, select_source
v4	has_hardware (live; a reader can come and go with a controller)

Frontend (nfc/)

A small dispatcher over pluggable backends:

• amiibo (default): serves dumps from the new Amiibo directory (recursive scan, non-amiibo-sized files skipped). Nothing auto-loads; a tag is placed by the core's v3 enumeration or the menu picker. Writes are flushed back to the dump, so in-game save-back persists.
• afnfc (--enable-nfc_afnfc, Linux): driver-agnostic hardware backend using only the kernel NFC generic-netlink family and AF_NFC raw sockets (what neard uses), so it works with any reader on the Linux NFC stack. It does not need or start neard. Tested with a pn533 USB reader and the Nintendo Switch Joy-Con reader.
• file (testing, RETRO_NFC_BACKEND=file): a single .bin, to exercise the core path without hardware.

Menu

• Settings > Directory > Amiibo. Defaults to <system>/amiibo.
• Quick Menu > Load Amiibo. A .bin picker using the normal settings file browser. Picking a dump places it on the virtual reader and returns straight to the game, since tapping an amiibo is an in-game action. The menu stack is flushed to the Quick Menu first, so reopening the menu lands there.
• Load Amiibo is shown only when the running core uses the NFC interface and the active source is software. When a core switches source while the menu is open, the current list is rebuilt immediately, so the entry appears or disappears without reopening the menu.

Both entries are compiled out without HAVE_NFC.

Commits

1. libretro: add NFC reader interface (GET_NFC_INTERFACE)
2. nfc: add NFC reader module with software, file and AF_NFC backends
3. runloop: wire GET_NFC_INTERFACE and track NFC-capable cores
4. build: add HAVE_NFC and HAVE_NFC_AFNFC options (also griffin)
5. menu: add Amiibo directory setting and Load Amiibo picker

Each commit builds on its own.

Testing

• Built on Linux at every commit, with and without --enable-nfc --enable-nfc_afnfc. NFC sources are C89-clean (-std=gnu89 -Wdeclaration-after-statement) and build in the griffin unity build.
• Software backend: enumeration, explicit placement, fresh-tap status sequence, read/write with flush-back, hardware fallback when no reader is present, and release on core unload.
• On hardware: Lakka on Nintendo Switch, with the Joy-Con NFC reader and a pn533 USB reader through the AF_NFC backend, and a 3DS core (Azahar) picking up amiibo through the software picker.

Notes for reviewers

• 99 was the next free experimental slot when this was written. Happy to renumber if something else lands first.
• The core-side consumer (3DS cores) will follow in their own repos once the interface shape is agreed.

[GavinDarkglider]

Sorry, I pushed v4, and actually added mifare support for Skylanders/disney infinity support in v5, then had Claude port the wrong version forward. Lol. Will rebase, and force push when it is done.