Split the schema-free half of the unit and geolocation helpers out

unit.h and geolocation.h both include ../ifcparse/express.h for the entity
walking they do, which puts the whole module out of reach of anything that
cannot link IfcParse. Most of what a viewer wants from them needs no IFC at
all: the unit conversion tables, and the Helmert parameters-to-matrix math.

Move those into unit_convert and geolocation_transform, and build them as a
new helpers_math target that `helpers` re-exports PUBLIC, so existing callers
keep working through the unchanged unit.h / geolocation.h includes. The new
target has no IfcParse or Qt dependency and so builds under Emscripten, where
the rest of this directory cannot.

One target rather than compiling the sources into each consumer: the glob in
this directory would otherwise put them in libhelpers.a as well, leaving two
copies of the same objects in any link that pulls both.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/helpers/geolocation_transform.h

@@ -0,0 +1,80 @@
+/********************************************************************************
+ *                                                                              *
+ * This file is part of IfcOpenShell.                                           *
+ *                                                                              *
+ * IfcOpenShell is free software: you can redistribute it and/or modify         *
+ * it under the terms of the Lesser GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3.0 of the License, or          *
+ * (at your option) any later version.                                          *
+ *                                                                              *
+ * IfcOpenShell is distributed in the hope that it will be useful,              *
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of               *
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the                 *
+ * Lesser GNU General Public License for more details.                          *
+ *                                                                              *
+ * You should have received a copy of the Lesser GNU General Public License     *
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.         *
+ *                                                                              *
+ ********************************************************************************/
+
+// Georeferencing matrix math over plain numbers: no IFC file, no schema
+// dispatch, no Qt.
+//
+// Split out of geolocation.h/geolocation.cpp for the same reason as
+// unit_convert: those pull in ../ifcparse/express.h to read an
+// IfcMapConversion out of a model, but IfcViewerCore — and through it the
+// Emscripten build — needs to compose the resulting matrices without
+// linking IfcParse.  Reading the parameters out of an IFC stays in
+// geolocation.h; turning them into matrices lives here.
+
+#ifndef GEOLOCATION_TRANSFORM_H
+#define GEOLOCATION_TRANSFORM_H
+
+#include <Eigen/Dense>
+
+struct HelmertTransformation {
+    double e = 0.0;        // eastings offset
+    double n = 0.0;        // northings offset
+    double h = 0.0;        // orthogonal-height offset
+    double xaa = 1.0;      // X-axis abscissa  (cos of grid-rotation angle)
+    double xao = 0.0;      // X-axis ordinate  (sin of grid-rotation angle)
+    double scale = 1.0;    // unit scale (project unit -> map unit)
+    double factor_x = 1.0; // combined scale factor along X
+    double factor_y = 1.0; // combined scale factor along Y
+    double factor_z = 1.0; // combined scale factor along Z
+};
+
+// Apply a Helmert transformation to a 4x4 local matrix.
+Eigen::Matrix4d local_to_global(const Eigen::Matrix4d& matrix,
+                                const HelmertTransformation& params);
+
+// Build the Helmert transformation as a meter-input / meter-output 4x4 matrix
+// directly from parsed parameters, bypassing auto_local_to_global's normalisation
+// step.  Used by callers that want a single per-model georef matrix to compose
+// with placement matrices at upload time.
+//
+// Result has shape:
+//     [ R_z(theta) · diag(fx, fy, fz)  | (e, n, h) · u_m ]
+//     [ 0                               | 1               ]
+//
+// `map_unit_to_meters` is derived by the caller from the IFC project length
+// unit and the authoritative IfcMapConversion.Scale.  Since this matrix takes
+// meter inputs from the geometry iterator, Scale is represented by that unit
+// conversion and is not applied again in the linear block.  The caller composes
+// any IfcGeometricRepresentationContext WCS on the right:
+//     G = helmert_meters_from_parameters(...) · inv(wcs_meters)
+// (where wcs_meters has its translation column converted from project units
+// to meters via calculate_unit_scale).
+//
+// Unlike auto_local_to_global, this preserves IfcMapConversionScaled.FactorX/Y/Z
+// in the rotation block, so they apply correctly to placement translations
+// when composing per-model.
+Eigen::Matrix4d helmert_meters_from_parameters(const HelmertTransformation& params,
+                                               double map_unit_to_meters);
+
+// "How do I rotate project east to get to grid east?" — i.e. -atan2(xao, xaa)
+// converted to degrees, anticlockwise positive.  Mirrors
+// ifcopenshell.util.geolocation.xaxis2angle.
+double x_axis_to_angle_deg(double xaa, double xao);
+
+#endif // GEOLOCATION_TRANSFORM_H