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/src/domain/niweleta.cpp

@@ -0,0 +1,90 @@
+#include "maj0sted/domain/niweleta.hpp"
+
+#include <cmath>
+#include <cstddef>
+#include <stdexcept>
+#include <utility>
+#include <variant>
+
+namespace maj0sted::domain {
+
+Niweleta::Niweleta(NiweletaId id, std::string name)
+    : id_{id}, name_{std::move(name)} {}
+
+StraightId Niweleta::next_straight_id() noexcept { return StraightId{++straight_seq_}; }
+
+StraightId Niweleta::add_straight(CartesianPosition start, CartesianPosition end,
+                                  JointContinuity joint) {
+    const StraightId id = next_straight_id();
+    plan_.append(Straight{id, start, end}, joint);
+    return id;
+}
+
+void Niweleta::add_plan_element(PlanElement element, JointContinuity joint) {
+    if (auto* straight = std::get_if<Straight>(&element); straight != nullptr) {
+        if (straight->id().is_null()) {
+            // A straight added without an identity gets one from the niweleta.
+            element = Straight{next_straight_id(), straight->start(), straight->end()};
+        } else if (straight->id().value() > straight_seq_) {
+            // Keep the counter ahead of any explicitly supplied id (e.g. on load).
+            straight_seq_ = straight->id().value();
+        }
+    }
+    plan_.append(std::move(element), joint);
+}
+
+void Niweleta::apply_fit(const FitResult& result) {
+    const StraightId entry_id = result.entry.id();
+    const StraightId exit_id = result.exit.id();
+    if (entry_id.is_null() || exit_id.is_null()) {
+        throw std::invalid_argument{"apply_fit: trimmed straights must be identified"};
+    }
+
+    const auto& elements = plan_.elements();
+    const auto index_of = [&](StraightId id) -> std::size_t {
+        for (std::size_t k = 0; k < elements.size(); ++k) {
+            const auto* straight = std::get_if<Straight>(&elements[k]);
+            if (straight != nullptr && !straight->id().is_null() &&
+                straight->id() == id) {
+                return k;
+            }
+        }
+        throw std::invalid_argument{"apply_fit: straight not found in plan"};
+    };
+    const std::size_t i = index_of(entry_id);
+    const std::size_t j = index_of(exit_id);
+    if (j != i + 1) {
+        throw std::invalid_argument{
+            "apply_fit: the two straights must be adjacent (entry then exit)"};
+    }
+
+    // Rebuild the plan, re-validating every joint via append().
+    HorizontalAlignment rebuilt;
+    const auto add = [&](PlanElement element, JointContinuity joint) {
+        rebuilt.append(std::move(element), joint);
+    };
+
+    for (std::size_t k = 0; k < i; ++k) {
+        add(elements[k], k == 0 ? JointContinuity::Tangent : plan_.joint(k - 1));
+    }
+    add(result.entry, i == 0 ? JointContinuity::Tangent : plan_.joint(i - 1));
+    for (const auto& curve_element : result.curve) {
+        add(curve_element, JointContinuity::Tangent);
+    }
+    add(result.exit, JointContinuity::Tangent);
+    for (std::size_t k = j + 1; k < elements.size(); ++k) {
+        add(elements[k], plan_.joint(k - 1));
+    }
+
+    plan_ = std::move(rebuilt);
+}
+
+void Niweleta::add_profile_element(ProfileElement element) {
+    profile_.append(std::move(element));
+}
+
+bool Niweleta::lengths_consistent(double tolerance_m) const {
+    return std::abs(plan_length().metres() - profile_length().metres()) <= tolerance_m;
+}
+
+}  // namespace maj0sted::domain