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, 9 insertions, 5 deletions

diff --git a/src/syscall.asm b/src/syscall.asm
index 870a2ff..cb2c996 100644
--- a/src/syscall.asm
+++ b/src/syscall.asm
@@ -122,17 +122,21 @@ KGetc::
     jp sys_getb                 ; B=fd -> A=byte, CF=EOF
 .console
 .poll
+    call pad_poll               ; poll joypad; drive the on-screen keyboard
+    call osk_handle             ; CF set + A = byte if a key was typed
+    jr c, .done
     ld a, $80
     ld [rSC], a
     ld a, [rSC]
     bit 7, a
-    jr z, .got
+    jr nz, .yield
+    ld a, [rSB]                 ; A = serial byte
+.done
+    and a                       ; CF = 0 (not EOF)
+    ret
+.yield
     call SchedYield
     jr .poll
-.got
-    ld a, [rSB]
-    and a                       ; CF = 0
-    ret
 
 ; -----------------------------------------------------------------------------
 ; KPutc(E = byte) - write to this process's stdout (console/file).