osk: toggle-able on-screen keyboard on the window layer

A joypad-driven keyboard that costs zero permanent screen space: it lives
on the GB window layer, so SELECT just flips LCDC bit 5 (window enable) and
the terminal's background layer underneath is never disturbed.

- src/joypad.asm: read $FF00 with edge detection (wPadCur/Prev/New).
- src/osk.asm: 3x20 key grid (letters, digits, space, punctuation, plus
  Enter/Backspace) built once into spare bank-1 VRAM tiles (104+) and laid
  out on the $9C00 window map, docked to the bottom 3 rows. The highlighted
  key is a palette swap on its window attribute byte (CGB BG palette 1 =
  inverted), so moving the cursor is 1-2 attribute writes with no tile
  rebuilding. SELECT toggles, d-pad moves, A types.
- KGetc's console poll loop now polls the joypad and osk_handle each
  iteration; a key press returns its byte to the reader exactly like a
  serial byte, so the shell is oblivious to the input source.

Verified in the emulator (via --keys): SELECT shows the keyboard, the
highlight tracks the d-pad, and typing "ls"+Enter runs the command and
lists the files; a second SELECT hides it and reclaims the full 18 rows.

Known limitation: the OSK overlays the bottom 3 terminal rows, so if the
prompt has scrolled to the very bottom the current input line can be
hidden. A follow-up can add a scroll region (terminal uses rows 0-14 while
the OSK is up). Input over serial still works unchanged.

1 files changed, 1 insertions, 0 deletions

diff --git a/include/gbos.inc b/include/gbos.inc
index c4804c4..92b82c9 100644
--- a/include/gbos.inc
+++ b/include/gbos.inc
@@ -11,6 +11,7 @@ DEF GBOS_INC EQU 1
 DEF rIF     EQU $FF0F   ; interrupt flags
 DEF rIE     EQU $FFFF   ; interrupt enable
 DEF rSVBK   EQU $FF70   ; CGB WRAM bank select ($D000-$DFFF): 1..7
+DEF wCurMask EQU $D589  ; term/osk: cursor underline bits OR'd into a tile's last row
 DEF rKEY1   EQU $FF4D   ; CGB speed switch
 DEF rSB     EQU $FF01   ; serial data
 DEF rSC     EQU $FF02   ; serial control