Space-time distorder drains energy while boosting (#62)

GameState._advance_distorder_drain() runs in GameState._process() -
not the distorder component's tick() - alongside extractor mining and
production, so it (and the fleet's travel) keeps working across the
space<->surface scene swap. Drain scales with how far above 1.0 the
multiplier is: (fleet_speed_multiplier - 1.0) / (MAX - 1.0) *
ENERGY_DRAIN_PER_SECOND (2.0 at full boost), accumulated fractionally
the same way extractor mining_progress is so small multipliers still
drain a nonzero amount instead of rounding to zero every frame.

Running the fleet hold's energy to exactly zero force-resets
fleet_speed_multiplier to 1.0 and raises a one-shot
boost_depleted_notice flag, since GameState can't reach into whichever
scene's SpeedFader may or may not currently exist. SpeedFader.gd
consumes the flag on its next _process(), snaps its knob down (it was
already reading the multiplier live, so this falls out for free) and
shows "Out of energy!" - also clearing active_touch_index so a
still-held drag can't immediately re-boost from the position the
finger never moved away from.

test_space_time_distorder.gd (#59) updated to stock fleet energy
first, for the same reason #60/#61 needed it: boosting is no longer
free, and that test's own multi-second boosted phases would otherwise
get cut short mid-measurement by running out.

GameState.gd

@@ -29,6 +29,17 @@ var planet_definition_id: String = "rocky"
 const MAX_FLEET_SPEED_MULTIPLIER: float = 3.0
 var fleet_speed_multiplier: float = 1.0
 
+# #62: energy cost of sustaining a boost, scaled by how far above 1.0 the
+# multiplier is - a small boost costs proportionally less, full boost costs
+# ENERGY_DRAIN_PER_SECOND. Sitting at exactly 1.0 costs nothing.
+const ENERGY_DRAIN_PER_SECOND: float = 2.0
+var _distorder_drain_progress: float = 0.0
+# One-shot notice, consumed and cleared by SpeedFader.gd: true the instant
+# a sustained boost is force-reset for running the fleet hold's energy
+# to zero, so the control (which may not even be part of the current
+# scene tree at that exact frame) can show why on its next _process().
+var boost_depleted_notice: bool = false
+
 
 func set_fleet_speed_multiplier(value: float) -> void:
 	fleet_speed_multiplier = clampf(value, 1.0, MAX_FLEET_SPEED_MULTIPLIER)
@@ -1165,6 +1176,32 @@ func _advance_gas_burners(delta: float) -> void:
 		b.gas_burn_progress = progress
 
 
+func _advance_distorder_drain(delta: float) -> void:
+	if fleet_speed_multiplier <= 1.0:
+		# Nothing owed at 1.0x - and no stale fractional progress should
+		# carry over into a LATER, unrelated boost.
+		_distorder_drain_progress = 0.0
+		return
+	var rate: float = (fleet_speed_multiplier - 1.0) / (MAX_FLEET_SPEED_MULTIPLIER - 1.0) \
+		* ENERGY_DRAIN_PER_SECOND
+	_distorder_drain_progress += rate * delta
+	var whole: int = int(floor(_distorder_drain_progress))
+	if whole <= 0:
+		return
+	_distorder_drain_progress -= whole
+	var available: int = fleet_inventory.amount(RESOURCE_ENERGY)
+	if available >= whole:
+		fleet_inventory.withdraw(RESOURCE_ENERGY, whole)
+		return
+	# Ran out mid-drain: take whatever is left (down to exactly zero) and
+	# force the boost back to its default, rather than let the fleet keep
+	# travelling fast on credit.
+	fleet_inventory.withdraw(RESOURCE_ENERGY, available)
+	fleet_speed_multiplier = 1.0
+	_distorder_drain_progress = 0.0
+	boost_depleted_notice = true
+
+
 # Planet conquest: a planet must have every one of its capture points held
 # before an extractor can be built there (see BuildController's placement
 # gates). Keyed by the same stable planet_id as `extractors`, entirely
@@ -1240,6 +1277,7 @@ func _process(delta: float) -> void:
 	_advance_lab_research(delta)
 	_advance_vessel_deliveries(delta)
 	_advance_gas_burners(delta)
+	_advance_distorder_drain(delta)
 	for e in extractors:
 		e.mining_progress = float(e.get("mining_progress", 0.0)) + delta * EXTRACTOR_RATE
 		var whole: int = int(floor(float(e.mining_progress)))