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>

GameController.gd

@@ -95,7 +95,6 @@ const KEY_PIVOT_CW: int = KEY_E
 const KEY_PIVOT_CCW: int = KEY_Q
 const WHEEL_ZOOM_STEP: float = 1.12
 
-@onready var keyboard_hint: Label = get_node_or_null("UI/KeyboardHintLabel")
 
 var keys_down: Dictionary = {}
 var keyboard_seen: bool = false
@@ -105,8 +104,6 @@ var keyboard_steering: bool = false
 func _ready() -> void:
 	if joystick != null:
 		joystick.direction_changed.connect(_on_joystick_direction)
-	if keyboard_hint != null:
-		keyboard_hint.visible = false
 
 
 func _held_pivot_direction() -> float:
@@ -150,8 +147,6 @@ func _note_keyboard() -> void:
 	if keyboard_seen:
 		return
 	keyboard_seen = true
-	if keyboard_hint != null:
-		keyboard_hint.visible = true
 	# The stick is only taken away where there is no touchscreen to work it
 	# with. A touch device that also has a keyboard keeps both, and a device
 	# wrongly reported as non-touch still shows the stick until the player