Vendor the maj0sted track layout library and link it into the editor

Drop-in copy of C:\Users\maj00r\Projects\maj0sted-master minus its web/
directory - the WASM frontend is not being ported, the editor drives the
same C++ API natively. What is left is plain C++20 with no dependencies:
domain model (niweleta, plan and profile geometry, fitting, parallelism),
project serialization, rail sampling and the editor document.

src/web/ribbon.cpp was missing from the library source list; added, since
without it the ribbon symbols never make it into the archive.

maj0sted/tests/test_render.cpp

@@ -0,0 +1,78 @@
+#include <cmath>
+
+#include "check.hpp"
+#include "maj0sted/io/svg_writer.hpp"
+#include "maj0sted/maj0sted.hpp"
+#include "maj0sted/render/plan_sampler.hpp"
+
+using namespace maj0sted::domain;
+
+namespace {
+
+void samples_one_polyline_per_element() {
+    Niweleta n{NiweletaId{1}};
+    const StraightId a =
+        n.add_straight(CartesianPosition{0.0, 0.0}, CartesianPosition{100.0, 0.0});
+    const StraightId b = n.add_straight(CartesianPosition{100.0, 0.0},
+                                        CartesianPosition{100.0, 100.0},
+                                        JointContinuity::AzimuthBreak);
+    const FitResult fit = FittingService::fit_between_straights(
+        n, FitParameters{.entry = a,
+                         .exit = b,
+                         .radius = Radius::from_metres(40.0),
+                         .transition_length = Length::from_metres(10.0)});
+    n.apply_fit(fit);  // straight -> KP -> arc -> KP -> straight (5 elements)
+
+    const auto scene = maj0sted::render::sample(n.plan());
+    CHECK(scene.polylines.size() == n.plan().size());
+    CHECK(scene.polylines.size() == 5);
+    CHECK(!scene.empty);
+
+    // First polyline is the (straight) entry, drawn from its anchored endpoints.
+    const auto& first = scene.polylines.front();
+    CHECK(first.kind == maj0sted::render::ElementKind::Straight);
+    CHECK(first.points.size() == 2);
+    CHECK(std::abs(first.points.front().x - 0.0) < 1e-9);
+
+    // Curves are sampled into many points.
+    CHECK(scene.polylines[2].kind == maj0sted::render::ElementKind::Arc);
+    CHECK(scene.polylines[2].points.size() > 2);
+}
+
+void curve_geometry_is_continuous() {
+    // The sampled points should join up (each element starts where the previous
+    // ended) for a properly fitted, tangent-continuous plan.
+    Niweleta n{NiweletaId{2}};
+    const StraightId a =
+        n.add_straight(CartesianPosition{0.0, 0.0}, CartesianPosition{100.0, 0.0});
+    const StraightId b = n.add_straight(CartesianPosition{100.0, 0.0},
+                                        CartesianPosition{100.0, 100.0},
+                                        JointContinuity::AzimuthBreak);
+    n.apply_fit(FittingService::fit_between_straights(
+        n, FitParameters{.entry = a, .exit = b, .radius = Radius::from_metres(40.0)}));
+
+    const auto scene = maj0sted::render::sample(n.plan());
+    for (std::size_t i = 1; i < scene.polylines.size(); ++i) {
+        const auto& prev_end = scene.polylines[i - 1].points.back();
+        const auto& next_start = scene.polylines[i].points.front();
+        CHECK(std::abs(prev_end.x - next_start.x) < 1e-6);
+        CHECK(std::abs(prev_end.y - next_start.y) < 1e-6);
+    }
+}
+
+void svg_is_produced() {
+    Niweleta n{NiweletaId{3}};
+    n.add_straight(CartesianPosition{0.0, 0.0}, CartesianPosition{50.0, 0.0});
+    const auto svg = maj0sted::io::to_svg(maj0sted::render::sample(n.plan()));
+    CHECK(svg.rfind("<svg", 0) == 0);
+    CHECK(svg.find("<polyline") != std::string::npos);
+}
+
+}  // namespace
+
+int main() {
+    RUN(samples_one_polyline_per_element);
+    RUN(curve_geometry_is_continuous);
+    RUN(svg_is_produced);
+    return REPORT();
+}