feat: lighter air control (steer + stronger strafe) and 2-charge dash

- Air: direct steering bends existing velocity toward input without changing
  its magnitude (air_steer_rate), on top of Quake accelerate with raised
  gains (air_strafe_accel 55->90, per-tick cap 1.2->2.5). No-input air drag
  nearly removed so held momentum carries. Steering skips opposing input to
  avoid flipping through zero.
- Dash: charge system on the machine — dash_charges (2) spend instantly,
  each recharges in dash_cooldown (2 s). get_dash_cooldown_remaining() now
  reports time to the NEXT charge (0 while one is banked) so existing HUDs
  keep working; get_dash_charges() added for pips.
- Tests: dash tests updated to the charge API + new coverage for
  spend-2-then-recharge; 11/11 pass, spawn smoke clean.

Co-Authored-By: Claude Fable 5 <[email protected]>

movement/states/state_ground.gd

@@ -126,7 +126,7 @@ func update(delta: float) -> void:
 					return
 
 	# ── Dash ──────────────────────────────────────────────────────────────
-	if machine.input_dash and machine.dash_cooldown_timer <= 0.0:
+	if machine.input_dash and machine.can_dash():
 		machine.switch_to("dash")
 		return