Sketch where the user pointed: a picked face is the sketch plane

Selecting a face and sketching on it is the most common gesture in solid
modelling, and it was impossible. The plane came from a combo holding
XY/XZ/YZ plus datums, and plane_from_choice had no face branch at all, so
the only route onto a face was to build a Coincident datum plane on it
first, confirm that, reopen the sketch and find the datum in the
dropdown. Three extra steps and a junk feature in the tree.

The fix is not another combo row. A new sketch now takes its plane from
what is SELECTED IN THE VIEWPORT: a picked planar face wins outright, and
only when nothing is picked does it fall back to the reference plane —
which is itself normally set by clicking one of the ghost planes in 3D,
not by opening the combo. The tool hint names the target ("Circle — click
center, then radius · on the picked face") so the choice is visible on the
geometry side rather than needing a control to read back.

CadDocument::plane_of_face is the shared derivation, so the sketch path
and the Coincident datum method cannot drift apart. It refuses
non-planar faces: face_normal_world evaluates at the mid parameter, which
on a cylinder or a fillet is a tangent plane at one arbitrary point —
fine for offsetting a datum, wrong as a sketch plane, and silently
sketching on a tangent is worse than declining.

Picking the face also CONSUMES it. Leaving the pick live meant the next
Extrude saw a selected face and push/pulled it instead of extruding the
sketch just drawn — the same trap the imported-art path already guards
against.

Verified on :10 end to end with no combo interaction: build a box, click
its top face twice to cycle whole -> face, press S then C, and the circle
is drawn in the plane of that face with its Radius tab on the geometry
(artifacts/shots/f3a2-03-face.png, f3a2-04-sketch-on-face.png,
f3a2-05-circle-drawn.png). Kernel side: 154 cases / 2125 assertions green
on both forks, including that a cylinder resolves exactly its two flat
caps and refuses the barrel.

Still side-panel-shaped and to be dealt with separately: the Plane combo
remains on the card and now merely displays a stale row when a face is
the real target. It should show the actual target or go away.

snaporca-3a2.

tests/libslic3r/test_caddocument.cpp

@@ -6798,3 +6798,58 @@ TEST_CASE("An entity sketch that forms no wire fails instead of extruding a defa
         CHECK(doc.error.find("do not form a single closed wire") != std::string::npos);
     }
 }
+
+// Sketching on a picked face is the most common gesture in solid modelling, and it was impossible:
+// the plane came from a combo of base + datum planes only, so the sole route onto a face was to
+// build a Coincident datum plane first. plane_of_face is the shared derivation that makes the
+// viewport selection usable directly. snaporca-3a2.
+TEST_CASE("plane_of_face gives a sketchable plane for a planar face only", "[CadDocument]")
+{
+    // 20 x 20 x 20 box on XY, so its top face sits at z = 20 with +Z normal.
+    CadDocument doc;
+    int sk = doc.add_sketch(SketchShape::Rectangle, SketchPlane::XY(), 20, 20, 10, "Sketch");
+    doc.add_extrude(sk, 20.0, false, BooleanMode::New, "Extrude");
+    REQUIRE(doc.recompute());
+    REQUIRE(doc.bodies.size() == 1);
+
+    SECTION("every planar face of the box resolves, and its normal is a unit axis") {
+        int resolved = 0, top = -1;
+        for (int fi = 0; fi < 64; ++fi) {
+            SketchPlane p;
+            if (!doc.plane_of_face(0, fi, p)) continue;
+            ++resolved;
+            CHECK_THAT(p.normal.norm(), Catch::Matchers::WithinAbs(1.0, 1e-9));
+            // The frame must be orthonormal or sketch coordinates on it would be skewed.
+            CHECK_THAT(p.x_axis.dot(p.y_axis), Catch::Matchers::WithinAbs(0.0, 1e-9));
+            CHECK_THAT(p.x_axis.dot(p.normal), Catch::Matchers::WithinAbs(0.0, 1e-9));
+            if (p.normal.z() > 0.99) top = fi;
+        }
+        CHECK(resolved == 6);                 // a box has exactly six planar faces
+        REQUIRE(top >= 0);                    // and one of them faces +Z
+        SketchPlane p;
+        REQUIRE(doc.plane_of_face(0, top, p));
+        CHECK_THAT(p.origin.z(), Catch::Matchers::WithinAbs(20.0, 1e-6));   // the top, not the base
+    }
+
+    SECTION("bad indices are refused rather than guessed") {
+        SketchPlane p;
+        CHECK_FALSE(doc.plane_of_face(0, -1, p));
+        CHECK_FALSE(doc.plane_of_face(-1, 0, p));
+        CHECK_FALSE(doc.plane_of_face(9, 0, p));
+        CHECK_FALSE(doc.plane_of_face(0, 999, p));
+    }
+
+    SECTION("a cylindrical face is refused — it has no single sketch plane") {
+        CadDocument cyl;
+        int c = cyl.add_sketch(SketchShape::Circle, SketchPlane::XY(), 0, 0, 10.0, "Sketch");
+        cyl.add_extrude(c, 20.0, false, BooleanMode::New, "Extrude");
+        REQUIRE(cyl.recompute());
+        int planar = 0, refused = 0;
+        for (int fi = 0; fi < 16; ++fi) {
+            SketchPlane p;
+            if (cyl.plane_of_face(0, fi, p)) ++planar; else ++refused;
+        }
+        CHECK(planar == 2);        // the two flat caps, and NOT the barrel
+        CHECK(refused > 0);
+    }
+}