Sharpen upgrades, mark the convoy's destination, and give the world sound

#204 the Beam Collimator widens the beam instead of adding range to one that
already crosses the screen; beam width becomes a per-shot vessel stat.
#205 the Energy Propulser gains three upgrades - dash recharge, dash reach,
and boosted-cruise drain - which turns three constants into vessel stats.
#206 three further doctrines carry the fleet from ten to twenty.
#207 the prologue's World Ship gets a marker, through the scanner's own
guidance arrow rather than a second one.
#208 #210 #213 #214 four new synthesised sounds: the breach opening, cargo
reaching the hold, a rock breaking, and a gas bomb blooming.
#215 building health bars are larger, plated and framed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019AJMdB7iuHuLFZM1aBu3GD

vfx/GasBombEffect.gd

@@ -66,6 +66,10 @@ func _advance_flight(delta: float) -> void:
 
 
 func _explode() -> void:
+	# #214: at the bloom, not at the launch - the throw is silent and the
+	# arrival is the event, the same way the beam's own noise sits at the
+	# muzzle rather than at the button.
+	Sfx.play(get_tree(), Sfx.GAS_BLAST)
 	exploded = true
 	explosion_time = EXPLOSION_DURATION
 	hit_count = 0