Implement #80, #81, #82 and #83

#80 - desktop combat. The mouse aims, the left button fires, and the number
keys pick the weapon: 1 the fleet's guns, 2 and up the abilities. The twin
sticks are built for thumbs; on a desktop they mean dragging a virtual stick
with a pointer that could simply BE the crosshair.

The pointer path hands its heading to FireStickController rather than driving
weapons itself, so exactly one thing in the scene aims a gun, and a held stick
wins - a finger on it is a deliberate act, a mouse resting somewhere is not.
Abilities gained fire_at(), which aims and fires in one call instead of
pretending to be a press-drag-release touch.

It is gated on the absence of a touchscreen, because Godot emulates a mouse
button from every touch and without that a tap on a phone would fire the guns.
The gate has an explicit override, and not for tidiness: headless reports a
touchscreen, so with the automatic answer alone the aiming could never be
exercised by a test and the only covered behaviour would be the guard.

#81 - the bottom-left control hint is gone, from both scenes and from
GameController.

#82 - a chaser is destroyed by the impact when it reaches a vessel. It used to
pass through the wreck untouched and line up another pass, which made a
ramming enemy strictly better than any other. Data-driven per enemy
(dies_on_contact), because a sniper that happens to drift into a ship has not
spent itself doing it - and routed through take_damage, so it dies exactly as
it dies to gunfire. The enemy is told about the impact BEFORE the ship
resolves its own damage, since resolving it can free the ship.

#83 - the research tree's buy button is gone; tapping a node twice buys it.
Deliberately "the same node twice" rather than a timed double-tap: no window
to miss on a slow phone, no accidental purchase while browsing, and the first
tap always does something visible, so the second is never a blind commitment.
The status line now says "Tap again to research" - with the button gone it is
the only thing distinguishing an affordable node from an unaffordable one.

test_enemy_collision asserted the exact behaviour #82 reversed, and now checks
the ram costs both. Suite 117/117.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

AimedFleetAbilityController.gd

@@ -59,6 +59,25 @@ func can_begin_aim() -> bool:
 		and not aiming and _next_ready_ship() != null
 
 
+# Fires immediately at a world point, with no press-drag-release. The mouse
+# path (#80) has a crosshair already - it does not need to be dragged into
+# existence - so it aims and fires in one call rather than pretending to be
+# a touch.
+func fire_at(world_target: Vector2) -> bool:
+	if not button.visible or button._blocked_by_modal() or _next_ready_ship() == null:
+		return false
+	if not _can_afford_shot():
+		_say(_unaffordable_message())
+		return false
+	# Recorded before firing because _fire() ends by restoring it, and a stale
+	# value from an earlier aim would snap the camera somewhere unrelated.
+	saved_zoom = camera.zoom.x
+	restoring_zoom = false
+	aim_target = world_target
+	_fire()
+	return true
+
+
 func handle_touch(event: InputEventScreenTouch) -> bool:
 	if event.pressed:
 		if not can_begin_aim() \