feat(ui): atlas overlay toggle bar — 9 MVP overlays + 2 locked (#836)

AtlasOverlayBar is an HBoxContainer docked top-right of the heightmap viewer
with 11 short-label buttons mapping to D-191 §7 / D-181 signal visibility:

  always-on (5)  TER INF NAM GAT POL — terrain, infrastructure, named
                 features, gate markers, political zones. Pinned on; clicks
                 are swallowed so the layers can't accidentally be disabled.
  toggleable (4) POP PRD SHD CRP — population density, production zones,
                 shadow economy, corporate presence. Reflect and mutate
                 viewer overlay state.
  locked (2)    STK BSL — stockpile_weeks, production_vs_baseline. Disabled
                 and greyed out with unlock-requirement tooltips, per D-181
                 semi-private/private tiers; kept in the bar so players see
                 that deeper data exists and is gated.

Each button writes through AtlasViewer.set_overlay_visible(), which is the
single entry point into the viewer's _overlay_visibility dict consumed by
AtlasMarkerOverlay._draw(). Locked overlays short-circuit in that setter.

Per D-191 criterion 6.

client/ui/implant/atlas_overlay_bar.gd

@@ -0,0 +1,101 @@
+class_name AtlasOverlayBar
+extends HBoxContainer
+
+## Overlay toggle bar for #835 AtlasViewer — nine MVP overlays plus two locked
+## layers (#836, D-191 §7, D-181 signal visibility ladder).
+##
+## Layout:
+##   [ALWAYS-ON]    terrain · infrastructure · named · gate · political
+##   [TOGGLEABLE]   pop density · production · shadow · corp presence
+##   [LOCKED]       stockpile weeks · production vs baseline
+##
+## Always-on buttons are visible but non-interactive (they render as pinned).
+## Toggleable buttons reflect viewer visibility state and flip on click.
+## Locked buttons are disabled, greyed, and carry a tooltip describing the
+## unlock requirement from D-181 — they are not removed from the bar so the
+## player can see that richer data exists and is gated.
+
+const OVERLAY_DEFS: Array = [
+	# id, label, group ("always" | "toggle" | "locked"), tooltip
+	{"id": "terrain",                "label": "TER", "group": "always", "tooltip": "Terrain — base heightmap. Always visible."},
+	{"id": "infrastructure",         "label": "INF", "group": "always", "tooltip": "Infrastructure — roads + rail. Always visible."},
+	{"id": "named_features",         "label": "NAM", "group": "always", "tooltip": "Named features — rivers, oceans, mountain ranges. Always visible."},
+	{"id": "gate_markers",           "label": "GAT", "group": "always", "tooltip": "Gate + spaceport markers. Always visible."},
+	{"id": "political_zones",        "label": "POL", "group": "always", "tooltip": "Political zones — currency-zone bands. Always visible."},
+	{"id": "population_density",     "label": "POP", "group": "toggle", "tooltip": "Population density (D-181 public)."},
+	{"id": "production_zones",       "label": "PRD", "group": "toggle", "tooltip": "Production zones (D-181 observable — requires presence)."},
+	{"id": "shadow_economy",         "label": "SHD", "group": "toggle", "tooltip": "Shadow economy zones (D-181 observable)."},
+	{"id": "corp_presence",          "label": "CRP", "group": "toggle", "tooltip": "Corporate presence — Tier 1 only (D-181 observable)."},
+	{"id": "stockpile_weeks",        "label": "STK", "group": "locked", "tooltip": "Stockpile weeks — LOCKED. Requires corporate contact (D-181 semi-private)."},
+	{"id": "production_vs_baseline", "label": "BSL", "group": "locked", "tooltip": "Production vs baseline — LOCKED. Requires insider access (D-181 private)."},
+]
+
+const COLOR_PINNED: Color = Color("#c8d0e0")
+const COLOR_ACTIVE: Color = Color("#f0d060")
+const COLOR_INACTIVE: Color = Color("#556677")
+const COLOR_LOCKED: Color = Color("#3a4a55")
+
+var _viewer = null
+var _buttons: Dictionary = {}  # overlay_id -> Button
+
+
+func _init(viewer_ref) -> void:
+	_viewer = viewer_ref
+	add_theme_constant_override("separation", 4)
+
+
+func _ready() -> void:
+	mouse_filter = Control.MOUSE_FILTER_STOP
+	for def: Dictionary in OVERLAY_DEFS:
+		var b: Button = Button.new()
+		b.text = def["label"]
+		b.tooltip_text = def["tooltip"]
+		b.toggle_mode = (def["group"] != "always")
+		b.focus_mode = Control.FOCUS_NONE
+		b.custom_minimum_size = Vector2(42.0, 24.0)
+		b.add_theme_font_size_override("font_size", 10)
+
+		match def["group"]:
+			"always":
+				b.disabled = false
+				b.button_pressed = true
+				b.add_theme_color_override("font_color", COLOR_PINNED)
+				b.add_theme_color_override("font_hover_color", COLOR_PINNED)
+				b.add_theme_color_override("font_pressed_color", COLOR_PINNED)
+				b.add_theme_color_override("font_disabled_color", COLOR_PINNED)
+				# Swallow clicks so the player can't accidentally turn terrain off,
+				# but keep it visible + tooltipped.
+				b.pressed.connect(_on_always_pressed.bind(def["id"]))
+			"toggle":
+				b.button_pressed = _viewer.is_overlay_visible(def["id"]) if _viewer else false
+				b.add_theme_color_override("font_color", COLOR_INACTIVE)
+				b.add_theme_color_override("font_pressed_color", COLOR_ACTIVE)
+				b.toggled.connect(_on_toggle_changed.bind(def["id"]))
+			"locked":
+				b.disabled = true
+				b.button_pressed = false
+				b.add_theme_color_override("font_disabled_color", COLOR_LOCKED)
+		add_child(b)
+		_buttons[def["id"]] = b
+
+
+func _on_always_pressed(overlay_id: String) -> void:
+	# Always-on overlays stay on. Keep the button visually pressed.
+	if _buttons.has(overlay_id):
+		_buttons[overlay_id].button_pressed = true
+
+
+func _on_toggle_changed(pressed: bool, overlay_id: String) -> void:
+	if _viewer:
+		_viewer.set_overlay_visible(overlay_id, pressed)
+
+
+func sync_from_viewer() -> void:
+	if _viewer == null:
+		return
+	for def: Dictionary in OVERLAY_DEFS:
+		if def["group"] != "toggle":
+			continue
+		var b: Button = _buttons.get(def["id"])
+		if b:
+			b.set_pressed_no_signal(_viewer.is_overlay_visible(def["id"]))

client/ui/implant/atlas_viewer.gd

@@ -101,6 +101,7 @@ var _canvas: Node2D = null                  # transformed node holding heightmap
 var _overlay_node: AtlasMarkerOverlay = null  # draws on top of heightmap
 var _city_panel = null                      # ImplantPanel sidebar (city data)
 var _empty_notice = null                    # ImplantPanel shown when heightmap missing
+var _overlay_bar: AtlasOverlayBar = null    # #836 overlay toggle bar (top-right)
 
 
 func _ready() -> void:
@@ -125,6 +126,7 @@ func _ready() -> void:
 
 	_build_city_panel()
 	_build_empty_notice()
+	_build_overlay_bar()
 
 
 ## Called by AtlasPanel when entering the viewer for a specific body.
@@ -524,9 +526,32 @@ func _position_empty_notice() -> void:
 	_empty_notice.position = Vector2((sz.x - 400.0) * 0.5, sz.y * 0.35)
 
 
+# =============================================================================
+# Overlay toggle bar (#836)
+# =============================================================================
+
+
+func _build_overlay_bar() -> void:
+	_overlay_bar = AtlasOverlayBar.new(self)
+	_overlay_bar.name = "OverlayBar"
+	add_child(_overlay_bar)
+	_position_overlay_bar()
+
+
+func _position_overlay_bar() -> void:
+	var sz: Vector2 = get_rect().size
+	if sz == Vector2.ZERO:
+		sz = Vector2(1280.0, 720.0)
+	# Top-right, leaving room for the header text
+	var bar_w: float = _overlay_bar.size.x if _overlay_bar.size.x > 0.0 else 520.0
+	_overlay_bar.position = Vector2(sz.x - bar_w - PANEL_MARGIN, PANEL_MARGIN)
+
+
 func _notification(what: int) -> void:
 	if what == NOTIFICATION_RESIZED:
 		if _city_panel:
 			_position_city_panel()
 		if _empty_notice:
 			_position_empty_notice()
+		if _overlay_bar:
+			_position_overlay_bar()