Fix cursor readout showing AZ/dist for points that are actually off the map

_on_cursor_move() only checked map bounds for the coord label itself
('off map' vs a real grid ref), the AZ/distance-from-nest numbers next
to it were computed and shown regardless, bearing_deg_point()/
distance_km_point() are happy to run on any raw km point, negative or
past the 20x10 grid included, so a cursor genuinely off the map could
still show what looked like a real reading. Now the whole label is
just 'off map', no numbers, whenever point_to_coord() says there's no
real grid position there.

src/fenigma/app.py

@@ -692,14 +692,21 @@ class MainWindow(Adw.ApplicationWindow):
             self.cursor_label.set_label("")
             return
         coord = solver.point_to_coord(point_km)
-        coord_text = coord.label() if coord is not None else "off map"
+        if coord is None:
+            # Off the map entirely, AZ/dist are still mathematically
+            # defined for any raw km point (nothing stops that math from
+            # running on a negative or >20/>10 coordinate), showing them
+            # anyway would read as a real reading for a position that
+            # isn't actually on the map.
+            self.cursor_label.set_label("off map")
+            return
         nest = self.board.nest
         if nest.coord is not None:
             az = ballistics.bearing_deg_point(nest.coord.as_fraction(), point_km)
             dist = ballistics.distance_km_point(nest.coord.as_fraction(), point_km)
-            self.cursor_label.set_label(f"AZ {az:05.1f}°  {dist:5.2f}km  {coord_text}")
+            self.cursor_label.set_label(f"AZ {az:05.1f}°  {dist:5.2f}km  {coord.label()}")
         else:
-            self.cursor_label.set_label(coord_text)
+            self.cursor_label.set_label(coord.label())
 
     # -- Nest -----------------------------------------------------------------
     def _build_nest_popover(self, rebuild) -> Gtk.Widget:

src/fenigma/grid_widget.py

@@ -7,6 +7,7 @@ stay in sync with the map (see app.py)."""
 from __future__ import annotations
 
 import math
+from collections import namedtuple
 
 import gi
 
@@ -29,6 +30,22 @@ LABEL_PAD = 8  # gap between a marker and its name label
 HOVER_RADIUS_PX = 12
 OVERLAY_RAY_LENGTH_KM = 30.0  # long enough to cross the 20x10 map from any origin
 
+MIN_ZOOM = 1.0  # the whole 20x10 map fits, the default
+MAX_ZOOM = 10.0
+ZOOM_STEP = 1.15  # per scroll-wheel notch
+
+# Everything needed to convert between km-space (the 20x10 grid) and
+# widget pixels for one frame, bundled so every draw/hit-test method
+# takes one argument instead of threading cell_w/cell_h/grid_h/pan
+# separately through a dozen call sites. cell_w/cell_h differ only when
+# square_cells is off (the default): the map then stretches to fill the
+# widget exactly, cell_w == cell_h only when the widget's own aspect
+# ratio happens to match the grid's. pad_x/pad_y are the letterboxing
+# margin added on whichever axis has leftover space when square_cells
+# is on. ox/oy are the visible viewport's origin in km-space (0,0
+# unless zoomed in and panned).
+_View = namedtuple("_View", "cell_w cell_h grid_w grid_h pad_x pad_y ox oy")
+
 CATEGORY_COLOR = {
     "nest": (0.35, 0.60, 0.95),
     "spotter": (0.35, 0.78, 0.40),