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>

BuildController.gd

@@ -161,7 +161,6 @@ var lab_building_id: int = 0
 @onready var skill_detail: Label = get_node("../UI/SkillMenu/DetailLabel")
 @onready var skill_cost: NeonNumber = get_node("../UI/SkillMenu/CostNumber")
 @onready var skill_status: Label = get_node("../UI/SkillMenu/StatusLabel")
-@onready var skill_unlock_button: IconButton = get_node("../UI/SkillMenu/UnlockButton")
 @onready var skill_close_button: IconButton = get_node("../UI/SkillMenu/CloseButton")
 
 @onready var formation_menu: Control = get_node("../UI/FormationMenu")
@@ -286,8 +285,8 @@ func _ready() -> void:
 	formation_editor.formation_changed.connect(_on_formation_changed)
 
 	skill_close_button.pressed.connect(func(): skill_menu.visible = false)
-	skill_unlock_button.pressed.connect(_on_unlock_pressed)
 	skill_tree.node_picked.connect(_on_skill_node_picked)
+	skill_tree.node_confirmed.connect(_on_skill_node_confirmed)
 
 	build_menu.visible = false
 	info_popup.visible = false
@@ -803,6 +802,13 @@ func _try_place(type: String, cost: int) -> void:
 
 # --- research (the mothership's skill tree) --------------------------------
 
+# A second tap on the node already selected buys it (#83). Routed into the
+# same handler the buy button used, so the affordability re-check and the
+# failure message it does are not duplicated here.
+func _on_skill_node_confirmed(_id: String) -> void:
+	_on_unlock_pressed()
+
+
 func _on_skill_node_picked(id: String) -> void:
 	_refresh_skill_menu(id)
 
@@ -817,7 +823,6 @@ func _refresh_skill_menu(id: String = "") -> void:
 		skill_cost.amount = 0
 		skill_status.text = "Fleet capacity %d · Research points %d" \
 			% [GameState.max_fleet_size(), GameState.research_points]
-		skill_unlock_button.visible = false
 		return
 
 	var node: Dictionary = GameState.skill_node(id)
@@ -831,25 +836,23 @@ func _refresh_skill_menu(id: String = "") -> void:
 
 	if GameState.is_unlocked(id):
 		skill_status.text = "Researched"
-		skill_unlock_button.visible = false
 	elif not GameState.prerequisite_met(id):
 		skill_status.text = "Needs %s first" % GameState.skill_node(node.requires).name
-		skill_unlock_button.visible = false
 	elif not GameState.can_afford(int(node.cost)):
 		# available_ore(), not the bank alone: research is paid for the same
 		# pooled way as everything else, so quoting the bank here would tell
 		# the player they cannot afford something they can.
 		skill_status.text = "Not enough rocks - you have %d" % GameState.available_ore()
-		skill_unlock_button.visible = false
 	elif GameState.research_points < points_needed:
 		# #63: a distinct reason from the ore one above - conflating them
 		# would leave the player trying to fix the wrong shortage.
 		skill_status.text = "Not enough research points - you have %d, need %d" \
 			% [GameState.research_points, points_needed]
-		skill_unlock_button.visible = false
 	else:
-		skill_status.text = ""
-		skill_unlock_button.visible = true
+		# The button that used to appear here was the only thing saying "this
+		# is buyable". With it gone the line has to say so itself, or an
+		# affordable node and an unaffordable one look identical.
+		skill_status.text = "Tap again to research"
 
 
 func _on_unlock_pressed() -> void: