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, 61 insertions, 0 deletions

diff --git a/src/joypad.asm b/src/joypad.asm
new file mode 100644
index 0000000..73283ac
--- /dev/null
+++ b/src/joypad.asm
@@ -0,0 +1,61 @@
+; =============================================================================
+; joypad.asm - read the Game Boy joypad ($FF00) with edge detection.
+; =============================================================================
+INCLUDE "include/gbos.inc"
+
+DEF rP1 EQU $FF00
+
+DEF PAD_A      EQU $01
+DEF PAD_B      EQU $02
+DEF PAD_SELECT EQU $04
+DEF PAD_START  EQU $08
+DEF PAD_RIGHT  EQU $10
+DEF PAD_LEFT   EQU $20
+DEF PAD_UP     EQU $40
+DEF PAD_DOWN   EQU $80
+
+SECTION "pad_state", WRAM0
+wPadCur::  DS 1       ; currently held (1 = pressed)
+wPadPrev:: DS 1       ; held on the previous poll
+wPadNew::  DS 1       ; newly pressed this poll (cur & ~prev)
+
+SECTION "pad", ROM0
+
+; read_joypad -> A = state, 1=pressed:
+;   bit0 A, 1 B, 2 Select, 3 Start, 4 Right, 5 Left, 6 Up, 7 Down
+read_joypad::
+    ld a, $20                   ; P14=0: select d-pad
+    ld [rP1], a
+    ld a, [rP1]
+    ld a, [rP1]                 ; let the lines settle
+    cpl
+    and $0F                     ; right/left/up/down (bits 0-3)
+    swap a                      ; -> bits 4-7
+    ld b, a
+    ld a, $10                   ; P15=0: select buttons
+    ld [rP1], a
+    ld a, [rP1]
+    ld a, [rP1]
+    ld a, [rP1]
+    ld a, [rP1]                 ; buttons bounce more; settle longer
+    cpl
+    and $0F                     ; A/B/Select/Start (bits 0-3)
+    or b
+    ld c, a
+    ld a, $30                   ; deselect both
+    ld [rP1], a
+    ld a, c
+    ret
+
+; pad_poll - refresh wPadCur/wPadPrev/wPadNew.
+pad_poll::
+    ld a, [wPadCur]
+    ld [wPadPrev], a
+    call read_joypad
+    ld [wPadCur], a
+    ld b, a
+    ld a, [wPadPrev]
+    cpl
+    and b                       ; newly pressed edges
+    ld [wPadNew], a
+    ret