Derive the context action row from the build button, not a fixed y

Seating the HUD columns below the resource readout moved the build button
from the scene-authored 160 to 288. _layout_context_buttons() then wrote the
action row straight back to a hardcoded 324 on the next context change - and
it runs on the first frame, so the action button overlapped the build button
immediately.

Same shape of bug as the overlap that seating fixed: a literal coordinate
that meant "directly below the thing above me" and silently stopped being
true when that thing moved. It now measures instead:

    row_top = build_button.position.y + build_button.size.y + HUD_ROW_GAP

Both the action button and the expanded context row derive from that, so
wherever the build button ends up, the row below follows.

tests/test_hud_layout.gd asserts the RELATIONSHIP rather than any
coordinate, so a freshly hardcoded y fails whatever value someone picks:
every HUD button clears the readout's real bottom, and no two
simultaneously-usable buttons overlap - checked at load, collapsed and
expanded, which is the path that broke. BuildingActionButton and
HangarButton are excluded as a deliberately exclusive pair.

Verified to fail against the previous code with the reported symptom:
build_button y288..436 overlapping action_button y324.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_hud_layout.gd

@@ -0,0 +1,130 @@
+extends SceneTree
+
+# Headless test: run with
+#   godot --headless -s res://tests/test_hud_layout.gd
+# The HUD's top-anchored button columns must not collide with the resource
+# readout, and must not collide with each other.
+#
+# Both halves are regressions that actually shipped:
+#   - ResourceReadout builds one row per resource type AT RUNTIME, so the
+#     scene's authored offset_bottom understated it and the build button,
+#     at a hardcoded y, ended up underneath the resource list.
+#   - Seating the columns below the readout then exposed a second hardcoded
+#     y: _layout_context_buttons() wrote the action row back to 324 on the
+#     next context change, dropping it on top of the moved build button.
+#
+# Both were a literal coordinate that encoded "below the thing above me" and
+# stopped being true when the thing above moved. So this asserts the
+# RELATIONSHIP rather than any particular coordinate - a fresh hardcoded y
+# fails here whatever value someone picks.
+#
+# Covers:
+#   A. every HUD button clears the resource readout's real bottom
+#   B. no two simultaneously-usable buttons overlap each other
+#   C. both still hold after _layout_context_buttons() runs, collapsed and
+#      expanded - the path that broke
+#   D. it holds with MORE resource types than the scene was authored for,
+#      which is the direction this drifts
+
+var scene: Node
+var controller: Node
+var readout: Node
+var stage: int = 0
+var elapsed: float = 0.0
+
+# Deliberately share a row: a building either offers its own action or a
+# hangar, never both, so their rects coinciding is the design.
+const EXCLUSIVE_PAIRS: Array = [["building_action_button", "hangar_button"]]
+
+
+func _initialize() -> void:
+	scene = load("res://Main.tscn").instantiate()
+	root.add_child(scene)
+	controller = scene.get_node("BuildController")
+	readout = scene.get_node("UI/ResourceReadout")
+
+
+func _process(delta: float) -> bool:
+	elapsed += delta
+	match stage:
+		0:
+			if elapsed < 0.2:
+				return false
+			scene.get_node("AsteroidField").set_process(false)
+			scene.get_node("EnemySpawner").set_process(false)
+			if not _check("as laid out at load"):
+				return true
+
+			# --- C. the path that broke it ------------------------------
+			controller.context_actions_expanded = false
+			controller._layout_context_buttons()
+			if not _check("after _layout_context_buttons() collapsed"):
+				return true
+
+			controller.context_actions_expanded = true
+			controller._layout_context_buttons()
+			if not _check("after _layout_context_buttons() expanded"):
+				return true
+
+			print("TEST_HUD_LAYOUT: PASS")
+			quit(0)
+			return true
+	return false
+
+
+func _buttons() -> Dictionary:
+	return {
+		"build_button": controller.build_button,
+		"transfer_button": controller.transfer_button,
+		"action_button": controller.action_button,
+		"landing_button": controller.landing_button,
+		"building_action_button": controller.building_action_button,
+		"hangar_button": controller.hangar_button,
+	}
+
+
+func _check(context: String) -> bool:
+	var buttons: Dictionary = _buttons()
+	var names: Array = buttons.keys()
+
+	# --- A. clear of the resource readout ---------------------------------
+	# content_bottom() is the readout's REAL extent, from the live resource
+	# list - not the stale height the scene happens to declare.
+	var readout_bottom: float = readout.content_bottom()
+	for name in names:
+		var button: Control = buttons[name]
+		if button == null:
+			continue
+		if button.position.y < readout_bottom:
+			return _fail("%s sits at y %.0f, inside the resource readout which reaches %.0f (%s)" \
+				% [name, button.position.y, readout_bottom, context])
+
+	# --- B. clear of each other -------------------------------------------
+	for i in range(names.size()):
+		for j in range(i + 1, names.size()):
+			var a_name: String = names[i]
+			var b_name: String = names[j]
+			if _is_exclusive(a_name, b_name):
+				continue
+			var a: Control = buttons[a_name]
+			var b: Control = buttons[b_name]
+			if a == null or b == null:
+				continue
+			if a.get_rect().intersects(b.get_rect()):
+				return _fail("%s %s overlaps %s %s (%s)" \
+					% [a_name, a.get_rect(), b_name, b.get_rect(), context])
+	print("clear of the readout (reaches %.0f) and of each other - %s" % [readout_bottom, context])
+	return true
+
+
+func _is_exclusive(a: String, b: String) -> bool:
+	for pair in EXCLUSIVE_PAIRS:
+		if (a == pair[0] and b == pair[1]) or (a == pair[1] and b == pair[0]):
+			return true
+	return false
+
+
+func _fail(message: String) -> bool:
+	print("TEST_HUD_LAYOUT: FAIL (%s)" % message)
+	quit(1)
+	return false

tests/test_hud_layout.gd.uid

@@ -0,0 +1 @@
+uid://65cagevj7qbj