Display board 1408: full output definition + firmware display content

New display-board-1408.md: pin-level definition of the board's outputs
(8 multiplexed hex digits via ICM7228B, isolated LPT LED monitor block,
no readback path) and, from the firmware disassembly, everything the RIO
puts on it: F0 boot banner, F1 test-mode banner (keypad sequence A-9-E
enters, D exits), and the sub-test displays — five live encoder-count
readouts, button test with input index, two lamp patterns, keypad test.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -92,12 +92,13 @@ hardware addressing is what the 0x00–0x6F logical input map in
   on an 8-line header (4×4 matrix). A second 74HCT164 drives the matrix
   rows through open-collector 74LS05 inverters; the 74HCT165 reads the four
   columns (`IN0-3`, 2.7K pull-ups) plus a board-detect line `BRD_DT*`.
-- **1408 Display board** (`DSP_1408`) — ICM7228B decoding `HD[00:04]` +
-  `AS0-2` + `WR` from the RIO's dedicated 14-pin *external display* port into
-  eight LTS313A digits (hex/code-B). The board also has a 26-pin **"IBM
-  printer port"** header with series resistors and per-line LEDs — a
-  diagnostic tap so a PC parallel port can exercise/observe the display bus.
-  Silkscreen reads "P/N 140B" but the schematic title block confirms **1408**.
+- **1408 Display board** (`DSP_1408`) — ICM7228B driving eight LTS313A
+  digits (hex decode) from the RIO's dedicated 14-pin *external display*
+  port; write-only, plus an isolated 26-pin **"IBM printer port"** LED
+  monitor block. Silkscreen reads "P/N 140B" but the schematic title block
+  confirms **1408**. Full pin-level output definition — and what the
+  firmware actually displays (boot banner + the keypad-A-9-E test mode) —
+  in [display-board-1408.md](display-board-1408.md).
 - **1413 Quad Amplifier Board** (`AMP_PLT`, silkscreen "Quad Amplifier
   Board … P/N 1413 REV. 1") — the actual amplification is a stock
   **Pyramid PB-150P "Pro Plus"** 4-channel 12 V car-audio amplifier bolted