Port: mirror emits a half that continues the chain

Carries snaporca 0231bd5b68. Parity holds: 17 files identical, 8 diverging as expected.

A reflection reverses orientation, so mirror_entities now hands the reflected half back reversed
in ORDER and flipped per ENTITY — an arc swapping its angles as well as its ends, a spline
reversing its control points. Appending it to the source then yields one walkable chain instead
of two halves meeting head-to-head, and a mirrored CCW loop stays CCW.

This is the producer half of the confusion that cost three defects; the consumers (offset, and
the exact loop area) keep their defensive handling, because that is what makes them correct for
hand-built and imported sketches rather than only for geometry this function produced.

Contract change, carried with the reason: a mirrored line's p0 is the reflection of the SOURCE's
p1, and a mirrored CCW arc keeps a POSITIVE sweep — the reflection negates it, walking it the
other way negates it again. Both [SketchEdit] cases updated, and a new [SketchProfile] case
"a mirrored half continues the original chain" pins the property directly.

Kernel here: all tests passed, 2687 assertions in 232 test cases. GUI target builds and links.

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

tests/libslic3r/test_sketchprofile.cpp

@@ -185,3 +185,25 @@ TEST_CASE("profile: a mirrored half offsets as one loop, not two", "[SketchProfi
         if (o.type == SketchEntity::Type::Arc)
             REQUIRE_THAT(o.radius, WithinAbs(R + d, 1e-9));
 }
+
+TEST_CASE("profile: a mirrored half continues the original chain", "[SketchProfile]")
+{
+    // The point of emitting the reflected half reversed: appending it to the source must give a
+    // chain you can WALK, head-to-tail, with no consumer having to notice that half of it came
+    // from a mirror. The end of the last source entity must be the start of the first mirrored
+    // one, and the end of the last mirrored one must close back to the very first start.
+    const std::vector<SketchEntity> half = {
+        line({0, -15}, {50, -15}), line({50, -15}, {50, 15}), line({50, 15}, {0, 15}) };
+    const auto m = SketchEngine::mirror_entities(half, Vec2d(0, 0), Vec2d(0, 1));
+    REQUIRE(m.size() == 3);
+
+    REQUIRE_THAT((half.back().p1 - m.front().p0).norm(), WithinAbs(0.0, 1e-9));
+    REQUIRE_THAT((m.back().p1   - half.front().p0).norm(), WithinAbs(0.0, 1e-9));
+
+    for (size_t i = 0; i + 1 < m.size(); ++i)
+        REQUIRE_THAT((m[i].p1 - m[i + 1].p0).norm(), WithinAbs(0.0, 1e-9));
+
+    auto all = half;
+    for (const auto& e : m) all.push_back(e);
+    REQUIRE(closed_wires(all) == 1);
+}