Add planet conquest state and gate extractor placement (#41)

Planets now need every capture point held before an extractor can go
up there: GameState tracks per-planet capture progress keyed by the
same stable planet_id as extractors, and BuildController's four
extractor placement checks (menu enable, status text, info popup,
confirm) all re-check conquest, not just proximity/duplication.

Part of the #40 conquest-mechanic breakdown.

GameState.gd

@@ -1020,6 +1020,66 @@ func _extractor_inventory(extractor: Dictionary) -> RefCounted:
 	return inventory
 
 
+# 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
+# separate from it - a planet can be conquered long before anyone builds an
+# extractor on it, or never get one at all.
+var planet_capture: Dictionary = {}
+
+signal planet_conquered(planet_id: String)
+
+
+func register_capture_points(planet_key: String, total: int) -> void:
+	# Idempotent: landing again on a planet must not wipe points already
+	# captured on a previous visit.
+	if planet_capture.has(planet_key):
+		return
+	var captured: Array = []
+	captured.resize(total)
+	captured.fill(false)
+	planet_capture[planet_key] = {"captured": captured, "conquered": false}
+
+
+func capture_state(planet_key: String) -> Dictionary:
+	return planet_capture.get(planet_key, {"captured": [], "conquered": false})
+
+
+func is_point_captured(planet_key: String, index: int) -> bool:
+	var captured: Array = capture_state(planet_key).get("captured", [])
+	if index < 0 or index >= captured.size():
+		return false
+	return bool(captured[index])
+
+
+func mark_point_captured(planet_key: String, index: int) -> void:
+	if not planet_capture.has(planet_key):
+		return
+	var state: Dictionary = planet_capture[planet_key]
+	var captured: Array = state.captured
+	if index < 0 or index >= captured.size():
+		return
+	captured[index] = true
+	if state.conquered:
+		return
+	if captured.all(func(c): return bool(c)):
+		state.conquered = true
+		planet_conquered.emit(planet_key)
+
+
+func is_planet_conquered(planet_key: String) -> bool:
+	return bool(capture_state(planet_key).get("conquered", false))
+
+
+func captured_point_count(planet_key: String) -> int:
+	var captured: Array = capture_state(planet_key).get("captured", [])
+	var count: int = 0
+	for c in captured:
+		if c:
+			count += 1
+	return count
+
+
 func _process(delta: float) -> void:
 	# Runs in the autoload, so mining and transport continue while the player
 	# is on a planet's surface or nowhere near either.