refactor(ui): atlas review warnings — headers, public API, KEY_N, overlay source of truth (#128)

4. AtlasPanel and AtlasViewer now compose their title + hint from an
   ImplantHeader child rather than hand-rolling them via draw_string, so the
   D-169 "theme swap changes the implant hardware appearance" invariant
   holds end-to-end. _refresh_screen_header() drives content per level and
   on system navigation.

5. AtlasViewer exposes city_canvas_pos(), get_hovered_city(),
   get_selected_city(), and get_overlay_defs() as public API — the marker
   overlay no longer reaches into underscore-prefixed state, which is
   especially important because viewer is an untyped var in the overlay.

6. KEY_N now consumes unconditionally while the viewer is visible, and
   main.gd's global economics-monitor toggle is gated on
   !HudGroups.is_app_active("implant/map/atlas"). Previously pressing N
   without a selected city fell through and closed the fullscreen atlas as
   a side effect.

7. OVERLAY_DEFS lives in AtlasViewer as the single source of truth.
   AtlasOverlayBar reads the list via viewer.get_overlay_defs(), and
   AtlasViewer derives _overlay_visibility / _overlay_locked from the same
   table at _ready() — no more hand-maintained parallel lists, so the bar
   and the guard in set_overlay_visible can't drift.

8. AtlasOverlayBar drops `class_name`: it now loads via
   load("res://ui/implant/atlas_overlay_bar.gd") from AtlasViewer, the same
   pattern AtlasPanel uses for AtlasViewer. _init(viewer_ref = null) keeps
   the required-arg footgun off the editor's introspection path.

9. `star-map-data` make target added to regenerate
   client/data/star_map_data.json from systems.db + wiki, and
   `check-star-map` wired into pre-pr-validate + pre-pr-client so any
   commit that touches the generator (or any downstream systems.db change
   like server #839) fails pre-pr until the JSON is regenerated. The
   terrain_reference data-availability dependency is no longer tribal
   knowledge.

Also addresses review #15 (push_warning on unknown overlay id in
set_overlay_visible) and #16 (disabled always-on buttons drop handler
churn) as part of the same refactor.

client/scripts/main.gd

@@ -185,8 +185,10 @@ func _unhandled_key_input(event: InputEvent) -> void:
 			if atlas_panel:
 				atlas_panel.toggle_visible()
 		elif event is InputEventKey and event.keycode == KEY_N:
-			# #824: N — toggle Economics Monitor implant panel (E is bound to interact)
-			if economics_panel:
+			# #824: N — toggle Economics Monitor implant panel (E is bound to interact).
+			# Gated on the atlas being inactive so the viewer's N → city-economics
+			# cross-link isn't shadowed by this global toggle (review #6).
+			if economics_panel and not HudGroups.is_app_active("implant/map/atlas"):
 				economics_panel.toggle_visible()
 		elif event is InputEventKey and event.keycode == KEY_BRACKETLEFT:
 			# #824: [ — cycle economics panel system selector backward