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.c

@@ -300,6 +300,33 @@ void player_update(Entity *self, float dt, const Tilemap *map) {
         return;
     }
 
+    /* ── Double-tap down for free downward jetpack ── */
+    if (!body->on_ground && input_pressed(ACTION_DOWN)) {
+        if (pd->down_tap_timer > 0) {
+            /* Second tap — trigger free downward dash */
+            pd->down_tap_timer = 0;
+            pd->dash_timer = PLAYER_DASH_DURATION;
+            pd->dash_dir   = vec2(0.0f, 1.0f);
+
+            /* Brief invincibility during dash */
+            pd->inv_timer = PLAYER_DASH_DURATION;
+            self->flags |= ENTITY_INVINCIBLE;
+
+            /* Jetpack burst downward */
+            Vec2 exhaust_pos = vec2(
+                body->pos.x + body->size.x * 0.5f,
+                body->pos.y + body->size.y * 0.5f
+            );
+            particle_emit_jetpack_burst(exhaust_pos, pd->dash_dir);
+            audio_play_sound(s_sfx_dash, 96);
+            return;
+        }
+        pd->down_tap_timer = 0.3f; /* window for second tap */
+    }
+    if (pd->down_tap_timer > 0) {
+        pd->down_tap_timer -= dt;
+    }
+
     if (input_pressed(ACTION_DASH) && pd->dash_charges > 0) {
         pd->dash_charges--;
         stats_record_dash();
@@ -478,6 +505,7 @@ void player_update(Entity *self, float dt, const Tilemap *map) {
 
     /* ── Landing detection ───────────────────── */
     if (body->on_ground && !pd->was_on_ground) {
+        pd->down_tap_timer = 0;  /* reset double-tap on landing */
         /* Just landed — emit dust at feet */
         Vec2 feet = vec2(
             body->pos.x + body->size.x * 0.5f,

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              */