docs: the ARNITECH_BLADE option and the mc64blade configuration

CONFIG.md

@@ -295,6 +295,12 @@ For the mc64 computers of arnitech only, whose expansion port holds the Arnitech
 
 Feature needs about 450 bytes in KERNAL segment, and the page at `$FF00`. Disable on any other machine.
 
+### `ARNITECH_BLADE`
+
+For the Blades of arnitech only, the `mc64blade` configuration: a 65816 in 6502 emulation mode with a C64's bank 0, both CIAs, the serial bus, and the Arnitech controller, but no VIC-II, SID, keyboard matrix, or tape. Requires `ARNITECH_CONTROLLER` and `IEC`. The screen editor, the keyboard matrix scan, the VIC-II set-up, and PAL detection are left out. The screen and the keyboard, devices 3 and 0, are the Terminal Card the controller finds by its type, `$04` (`src/kernal/arnitech/arnitech_terminal.s`): CHROUT writes each byte to it, as a glyph while quote or insert mode is on, CHRIN echoes the keys typed and on RETURN reads back the logical line under the cursor, the IRQ moves its keys into the keyboard buffer and its RUN/STOP into `STKEY`, `SCREEN` answers 80x25, and `PLOT` moves its cursor; without one, bytes printed go nowhere and no key comes. The serial bus waits its 1 ms on CIA #1's timer B. CINT puts handlers for the 65816's native-mode interrupts into the RAM under the KERNAL at `$FFE4-$FFEF` (`arnitech_native.s`), and BASIC's cold start loads `"*"` from device 8 and RUNs it before `READY.`, unless RUN/STOP is held (`arnitech_autoboot.s`).
+
+Feature saves about 1,600 bytes in KERNAL segment, and needs about 15 in BASIC segment. Disable on any other machine.
+
 ## Eye candy
 
 ### `COLORS_BRAND`