feat: add topography surface and cross-section tools to the 3D viewer

Add "Add Topography Surface" and "Colour Topography by Stratigraphic
Column" to the visualisation panel: samples the DEM across the model
extent, shades it by elevation by default, and can instead colour it
by evaluating the stratigraphic column at each point (via
model_manager.evaluate_stratigraphy_on_points +
stratigraphic_ids_to_rgb), with lighting disabled on that mode so
colours read as true/flat rather than shaded by DEM slope. Both the
DEM sampling and the stratigraphy evaluation run on a background
thread via background_task.py, matching the pattern used by the other
map2loop-backed widgets.

Add plane and line-extrusion cross-section tools (cross_section_utils
build_plane_mesh/build_line_extrusion_mesh), coloured by the same
stratigraphic-column pipeline. Thread data_manager through
LoopWidget -> VisualisationWidget -> FeatureListWidget so the
cross-section tools have CRS-aware access to project layers.

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

Author: lachlangrose

loopstructural/gui/loop_widget.py

@@ -48,7 +48,11 @@ def __init__(
         )
 
         self.visualisation_widget = VisualisationWidget(
-            self, mapCanvas=self.mapCanvas, logger=self.logger, model_manager=self.model_manager
+            self,
+            mapCanvas=self.mapCanvas,
+            logger=self.logger,
+            data_manager=self.data_manager,
+            model_manager=self.model_manager,
         )
         tabWidget.addTab(self.modelling_widget, "Modelling")
         tabWidget.addTab(self.visualisation_widget, "Visualisation")

loopstructural/gui/visualisation/cross_section_utils.py

@@ -0,0 +1,71 @@
+"""Pure-geometry helpers for building cross-section meshes.
+
+No Qt/pyvista-viewer or QGIS dependency -- callers (e.g. FeatureListWidget)
+sample points from the returned meshes and colour them via
+`mesh_scalar_utils.stratigraphic_ids_to_rgb`.
+"""
+
+import numpy as np
+import pyvista as pv
+
+
+def build_plane_mesh(origin, normal, size: float, resolution: int = 50) -> pv.PolyData:
+    """Build a square plane mesh centred at `origin`, oriented by `normal`.
+
+    Parameters
+    ----------
+    origin : array_like
+        (3,) plane centre.
+    normal : array_like
+        (3,) plane normal; does not need to be unit length.
+    size : float
+        Edge length of the (square) plane.
+    resolution : int, optional
+        Number of sample points along each edge, by default 50.
+    """
+    return pv.Plane(
+        center=origin,
+        direction=normal,
+        i_size=size,
+        j_size=size,
+        i_resolution=resolution,
+        j_resolution=resolution,
+    )
+
+
+def resample_polyline(coords_xy, n: int) -> np.ndarray:
+    """Resample a 2D polyline to `n` points, evenly spaced by arc length."""
+    coords = np.asarray(coords_xy, dtype=float)
+    seg_lengths = np.hypot(*np.diff(coords, axis=0).T)
+    cumulative = np.concatenate([[0.0], np.cumsum(seg_lengths)])
+    total_length = cumulative[-1]
+    if total_length == 0:
+        return np.repeat(coords[:1], n, axis=0)
+    targets = np.linspace(0.0, total_length, n)
+    x = np.interp(targets, cumulative, coords[:, 0])
+    y = np.interp(targets, cumulative, coords[:, 1])
+    return np.column_stack([x, y])
+
+
+def build_line_extrusion_mesh(
+    coords_xy, z_min: float, z_max: float, resolution: int = 50, z_resolution: int = 50
+) -> pv.StructuredGrid:
+    """Build a vertical cross-section by extruding a 2D line through a Z range.
+
+    Parameters
+    ----------
+    coords_xy : array_like
+        (M, 2) ordered line vertices, in the model's coordinate system.
+    z_min, z_max : float
+        Vertical extent of the extrusion.
+    resolution : int, optional
+        Number of points sampled along the line (by arc length), by default 50.
+    z_resolution : int, optional
+        Number of points sampled vertically, by default 50.
+    """
+    xy = resample_polyline(coords_xy, resolution)
+    z = np.linspace(z_min, z_max, z_resolution)
+    xx = np.tile(xy[:, 0][:, None], (1, z_resolution))
+    yy = np.tile(xy[:, 1][:, None], (1, z_resolution))
+    zz = np.tile(z[None, :], (resolution, 1))
+    return pv.StructuredGrid(xx, yy, zz)