The 4KB screen buffer (80x25x2 bytes) was allocated from near heap via
calloc(), which exhausted the 64KB data segment on 16-bit DOS. Now uses
_fcalloc()/_ffree() from OpenWatcom's far heap on 16-bit, keeping the
buffer outside DGROUP.
The TUI now works fully on 16-bit FreeDOS: full-screen editor, F-key
bar, cursor positioning, and scroll -- all via BIOS INT 10h through the
DOS HAL, with the screen buffer in far memory.
Changes:
- tui.h: GW_FAR macro (expands to __far on 16-bit, nothing elsewhere),
tui.screen declared as tui_cell_t GW_FAR *
- tui.c: _fcalloc/_ffree for 16-bit, _fmemmove for scroll_up()
- TUI_CELL() macro works unchanged (far pointer dereference is
transparent)
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