feat(v4.2): shared topology + precise tolerance system

- make_box: shared 8 vertices + edge dedup (was 24 vertices, now 8)
- ToleranceConfig: per-operation tolerances, global config
- fuzzy_equal/zero/gt/lt/gte/lte with absolute+relative tolerance
- fuzzy vector/point/parallel/perpendicular helpers
- adaptive_tolerance: scales with model size
- 14 tolerance tests

src/CMakeLists.txt

@@ -153,6 +153,7 @@ add_library(vde_brep STATIC
     brep/incremental_update.cpp
     brep/large_assembly.cpp
     brep/format_io.cpp
+    brep/tolerance.cpp
     brep/modeling.cpp
     brep/brep_drawing.cpp
     brep/step_export.cpp

src/brep/tolerance.cpp

@@ -0,0 +1,25 @@
+#include "vde/brep/tolerance.h"
+#include "vde/brep/brep.h"
+
+namespace vde::brep {
+
+// Global tolerance config
+static ToleranceConfig g_global_tolerance;
+
+const ToleranceConfig& ToleranceConfig::global() {
+    return g_global_tolerance;
+}
+
+void ToleranceConfig::set_global(const ToleranceConfig& cfg) {
+    g_global_tolerance = cfg;
+}
+
+double model_tolerance(const BrepModel& body) {
+    auto bb = body.bounds();
+    if (bb.min().x() > bb.max().x()) return 1e-6;
+
+    double extent = bb.extent().norm();
+    return adaptive_tolerance(extent);
+}
+
+} // namespace vde::brep