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>

Main.tscn

@@ -1,4 +1,4 @@
-[gd_scene load_steps=30 format=3]
+[gd_scene load_steps=31 format=3]
 
 [ext_resource type="Script" path="res://Starfield.gd" id="1"]
 [ext_resource type="Script" path="res://Fleet.gd" id="2"]
@@ -24,6 +24,7 @@
 [ext_resource type="Script" path="res://ResourceBar.gd" id="24"]
 [ext_resource type="Script" path="res://VesselDeliveryField.gd" id="26"]
 [ext_resource type="Script" path="res://ResourceTransferList.gd" id="27"]
+[ext_resource type="Script" path="res://MouseCombatController.gd" id="mouse_ctrl"]
 [ext_resource type="Script" path="res://FireStickController.gd" id="fire_ctrl"]
 [ext_resource type="Script" path="res://GasBombController.gd" id="29"]
 [ext_resource type="Script" path="res://LaserController.gd" id="30"]
@@ -62,6 +63,9 @@ script = ExtResource("29")
 [node name="FireStickController" type="Node2D" parent="."]
 script = ExtResource("fire_ctrl")
 
+[node name="MouseCombatController" type="Node2D" parent="."]
+script = ExtResource("mouse_ctrl")
+
 [node name="LaserController" type="Node2D" parent="."]
 script = ExtResource("30")
 
@@ -203,19 +207,6 @@ offset_bottom = 172
 script = ExtResource("10")
 icon = 0
 
-[node name="KeyboardHintLabel" type="Label" parent="UI"]
-visible = false
-mouse_filter = 2
-anchor_top = 1.0
-anchor_bottom = 1.0
-offset_left = 28
-offset_right = 360
-offset_top = -300
-offset_bottom = -120
-theme_override_font_sizes/font_size = 36
-autowrap_mode = 2
-text = "WASD / arrows  move      Q / E  pivot      wheel  zoom"
-
 [node name="Joystick" type="Control" parent="UI"]
 mouse_filter = 2
 script = ExtResource("13")
@@ -2140,19 +2131,6 @@ autowrap_mode = 2
 horizontal_alignment = 1
 text = ""
 
-[node name="UnlockButton" type="Control" parent="UI/SkillMenu"]
-anchor_left = 0.5
-anchor_top = 1.0
-anchor_right = 0.5
-anchor_bottom = 1.0
-offset_left = -74.0
-offset_top = -170.0
-offset_right = 74.0
-offset_bottom = -22.0
-script = ExtResource("10")
-icon = 5
-glow_color = Color(0.45, 0.9, 0.55, 1)
-
 [node name="CloseButton" type="Control" parent="UI/SkillMenu"]
 anchor_left = 1.0
 anchor_right = 1.0