Add free downward jetpack on double-tap down mid-air

Double pressing the down arrow while airborne triggers a free downward
dash that doesn't consume jetpack charges. Uses a 0.3s window for the
double-tap detection, resets on landing.

Closes #9

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/game/player.h

@@ -67,6 +67,8 @@ typedef struct PlayerData {
     AimDir  aim_dir;               /* current aim direction         */
     bool    looking_up;            /* holding up without moving     */
     float   look_up_timer;         /* how long up has been held     */
+    /* Down-arrow double-tap (free downward jetpack) */
+    float   down_tap_timer;        /* time since last mid-air down press  */
     /* Death / Respawn */
     float   respawn_timer;         /* countdown after death anim finishes */
     Vec2    spawn_point;           /* where to respawn              */