Map: draw the fine 10x10 subdivision grid for the cell the cursor is in

Tracks the hovered large cell (col, row) separately from marker
hit-testing, since it needs to update on any cursor motion, not just
when crossing a marker's hit radius, redrawn only when the actual cell
changes, not on every pixel of motion within the same one. Lines drawn
inside the existing clip region so they never bleed past the visible
viewport at any zoom/pan state, dimmer than the main grid lines
(0.6x alpha, tuned down further after an initial too-loud pass) so
they read as a subtle aid, not competing visual noise.

Verified with a rendered screenshot: the subdivision lines appear only
within the hovered cell, correctly clipped, and stay faint against the
main grid.

src/fenigma/grid_widget.py

@@ -117,6 +117,12 @@ class GridCanvas(Gtk.DrawingArea):
         self.on_right_click = None  # callback(Coord, x, y), fired on right-click (unless placing)
         self.hide_dead_from_map = False  # off by default; toggled from the firing panel toolbar
 
+        # Which large cell the cursor is currently over, (col, row) both
+        # floored, or None off the map/off the widget entirely. Redrawn
+        # only when this actually changes cell (not on every pixel of
+        # motion within the same one), see _draw_hover_subgrid().
+        self._hover_cell: tuple[int, int] | None = None
+
         # Placement mode: while armed, the next left-click calls
         # placement_callback(Coord) instead of doing the normal
         # select/hit-test, and (if placement_preview_radius_km is set) a
@@ -423,6 +429,12 @@ class GridCanvas(Gtk.DrawingArea):
         if self.on_cursor_move is not None:
             self.on_cursor_move(cursor_km)
 
+        col, row = cursor_km
+        new_cell = (int(col), int(row)) if (0 <= col < COLS and 0 <= row < ROWS) else None
+        if new_cell != self._hover_cell:
+            self._hover_cell = new_cell
+            self.queue_draw()
+
         if self.placement_callback is not None:
             self._placement_cursor_km = cursor_km
             self.queue_draw()
@@ -440,6 +452,9 @@ class GridCanvas(Gtk.DrawingArea):
         self._last_pointer_px = None
         if self.on_cursor_move is not None:
             self.on_cursor_move(None)
+        if self._hover_cell is not None:
+            self._hover_cell = None
+            self.queue_draw()
         if self.placement_callback is not None:
             self._placement_cursor_km = None
             self.queue_draw()
@@ -552,6 +567,7 @@ class GridCanvas(Gtk.DrawingArea):
         cr.rectangle(MARGIN_LEFT + view.pad_x, MARGIN_TOP + view.pad_y, view.grid_w, view.grid_h)
         cr.clip()
 
+        self._draw_hover_subgrid(cr, view)
         self._draw_geo_overlays(cr, view)
         self._draw_firing_arrows(cr, view)
         self._draw_blast_radius(cr, view)
@@ -600,6 +616,29 @@ class GridCanvas(Gtk.DrawingArea):
 
         cr.restore()
 
+    def _draw_hover_subgrid(self, cr, view) -> None:
+        """The fine 10x10 x:y subdivision lines for whichever large
+        cell the cursor is currently over, for precise sub-cell
+        targeting, most useful once zoomed in enough that a single
+        large cell actually has room to show them meaningfully."""
+        if self._hover_cell is None:
+            return
+        cell_col, cell_row = self._hover_cell
+        r, g, b, a = GRID_LINE
+        cr.set_source_rgba(r, g, b, a * 0.6)  # fainter than the plain grid lines, not louder
+        cr.set_line_width(1)
+        x0, y0 = self._km_to_px(view, (cell_col, cell_row))
+        x1, y1 = self._km_to_px(view, (cell_col + 1, cell_row + 1))
+        for i in range(1, 10):
+            x, _ = self._km_to_px(view, (cell_col + i / 10, cell_row))
+            cr.move_to(x, y0)
+            cr.line_to(x, y1)
+        for i in range(1, 10):
+            _, y = self._km_to_px(view, (cell_col, cell_row + i / 10))
+            cr.move_to(x0, y)
+            cr.line_to(x1, y)
+        cr.stroke()
+
     def _draw_marker(self, cr, view, point_km, color, label,
                       canvas_width, canvas_height, *, hollow=False, dim=False,
                       selected=False, coord=None, extra_line=None, icon_surface=None) -> None: