Phase 7: DRAW screen + 3-digit counter

Add draw.c/draw.h with draw_render_counter(value): renders a 0..999
counter as 3 seven-segment-style digits, each 3 cells wide x 8 cells
tall (1 cell per segment), centered on the white DRAW screen at row 8,
columns 15-23. On segments write 0xFF bitmap + color RAM 0 (black);
off segments write 0x00 bitmap (white from $D021).

Wire into game.c:
- game_enter_draw resets draw_counter = 0 and calls draw_render_counter(0).
- game_step_draw increments draw_counter (cap 999) and re-renders each
  frame, before the existing fire-detection and fault logic.
- Flash effect: for the first 4 frames of DRAW, $D020 strobes between
  black and white, then settles to white (matches the white screen).

No audio or other behavior changes.

src/draw.h

@@ -0,0 +1,20 @@
+#ifndef WHACK_HARE_DRAW_H
+#define WHACK_HARE_DRAW_H
+
+// draw.h — DRAW screen counter rendering.
+//
+// The DRAW screen is solid white (set up by show_white_screen).
+// This header exposes the per-frame counter renderer, which writes
+// 3 seven-segment-style digits (each 3 cells wide × 8 cells tall,
+// 1 cell per segment) into the bitmap at row 8, columns 15-23.
+//
+// Public API:
+//   draw_render_counter(value) — render the 0..999 value as a
+//     3-digit counter on the white screen.  Values outside 0..999
+//     are clamped.
+
+void draw_render_counter(int value);
+
+#pragma compile("draw.c")
+
+#endif