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>

tests/test_enemy_collision.gd

@@ -2,9 +2,14 @@ extends SceneTree
 
 # Headless gameplay test: run with
 #   godot --headless -s res://tests/test_enemy_collision.gd
-# Verifies that an enemy touching a ship destroys that ship, is removed
-# from the fleet, while the enemy itself survives the touch (enemies only
-# die from being shot, not from ramming).
+# Verifies that an enemy touching a ship destroys that ship and takes it out
+# of the fleet.
+#
+# This used to also assert the enemy SURVIVED the touch. #82 reversed that for
+# the chaser - which is the enemy this spawns - so a ram now costs both. The
+# rule itself, including which enemies it applies to and what a shield does
+# about it, is tests/test_chaser_rams.gd; here it is only checked far enough
+# to confirm this test is exercising the ramming enemy it thinks it is.
 
 const CHECK_DELAY: float = 0.05
 
@@ -58,13 +63,14 @@ func _finish() -> void:
 	var ship_destroyed: bool = not is_instance_valid(target_ship)
 	var ship_removed_from_fleet: bool = fleet.ships.find(target_ship) == -1
 	var fleet_count_ok: bool = fleet.ships.size() == 4
-	var enemy_survived: bool = is_instance_valid(enemy) and enemy.hp > 0
+	# The chaser is destroyed by the impact too (#82).
+	var enemy_died: bool = not is_instance_valid(enemy) or enemy.is_queued_for_deletion()
 
-	print("ship_destroyed=%s ship_removed_from_fleet=%s ships_remaining=%d enemy_survived=%s" % [
-		ship_destroyed, ship_removed_from_fleet, fleet.ships.size(), enemy_survived
+	print("ship_destroyed=%s ship_removed_from_fleet=%s ships_remaining=%d enemy_died=%s" % [
+		ship_destroyed, ship_removed_from_fleet, fleet.ships.size(), enemy_died
 	])
 
-	if ship_destroyed and ship_removed_from_fleet and fleet_count_ok and enemy_survived:
+	if ship_destroyed and ship_removed_from_fleet and fleet_count_ok and enemy_died:
 		print("TEST_ENEMY_COLLISION: PASS")
 		quit(0)
 	else: