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>

SkillTreeView.gd

@@ -30,6 +30,18 @@ const LOCKED_PULSE: float = 0.5
 @export var node_radius: float = 46.0
 @export var row_spacing: float = 250.0
 
+# #83: a second tap on the node already selected buys it. The separate buy
+# button is gone - it sat at the bottom of the panel, a long way from the node
+# being chosen, and meant every purchase was look-down-tap-look-up. Tapping
+# the thing you want is the shorter sentence.
+#
+# Deliberately "tap the SAME node twice" rather than a timed double-tap: no
+# window to miss on a slow phone, no accidental purchase from a fast
+# double-tap while browsing, and the first tap always does something visible
+# (it selects, and the panel fills in with the cost and what it unlocks) so
+# the second one is never a blind commitment.
+signal node_confirmed(id: String)
+
 var selected_id: String = ""
 var scroll_y: float = 0.0
 var _dragging: bool = false
@@ -90,9 +102,12 @@ func handle_touch(event: InputEventScreenTouch) -> bool:
 	var local: Vector2 = event.position - global_position
 	for node in GameState.SKILL_TREE:
 		if local.distance_to(node_screen_pos(node)) <= node_radius * 1.25:
+			var confirming: bool = selected_id == node.id
 			selected_id = node.id
 			queue_redraw()
 			node_picked.emit(node.id)
+			if confirming:
+				node_confirmed.emit(node.id)
 			return true
 	return false