fix(stereo3d): ревью метода следов — центрирование следа, фикс скрытия сечения

- _traceLine: p0 = основание перпендикуляра из начала координат (след
  рисуется у фигуры, а не у далёкого пересечения с осью)
- фикс: после сброса/смены фигуры в пошаговом режиме step мог стать 0 →
  сечение скрыто и шаги не рисуются; нормализация step≥1 в _drawSection3P
- подпись шага обновляется сразу после 3-го клика (в step-режиме)
- bump stereo.js?v=10

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/js/labs/stereo.js

@@ -583,7 +583,9 @@ class StereoSim {
   clearSection3P() {
     this._section3PPicks = [];
     this._section3PData  = null;
-    this._section3PStep  = 0;
+    // keep step ≥1 while in step mode, else a re-pick would hide the section
+    this._section3PStep  = this._section3PStepBy ? 1 : 0;
+    this._stepCaption    = '';
     this._clearGroup(this._section3PGroup);
     this._notify();
   }
@@ -2265,6 +2267,9 @@ class StereoSim {
 
     if (!data || picks.length < 3) return;
 
+    // In step mode the step counter must be ≥1 (any reset path may have zeroed it),
+    // otherwise the section would be hidden and no step drawn.
+    if (this._section3PStepBy && this._section3PStep < 1) this._section3PStep = 1;
     // In step-by-step mode the finished section is hidden until step ≥ 5 so the
     // trace construction builds up; otherwise (or at the end) show it in full.
     const showFull = !this._section3PStepBy || this._section3PStep >= 5;
@@ -2353,11 +2358,11 @@ class StereoSim {
   // when the cutting plane is (nearly) parallel to the base (trace at infinity).
   _traceLine(data) {
     const a = data.normal.x, b = data.normal.z, D = data.D;
-    if (Math.abs(a) < 1e-6 && Math.abs(b) < 1e-6) return null; // parallel to base
+    const denom = a * a + b * b;
+    if (denom < 1e-12) return null;                       // plane parallel to base
     const dir = new THREE.Vector3(-b, 0, a).normalize();
-    let p0;
-    if (Math.abs(a) >= Math.abs(b)) p0 = new THREE.Vector3(-D / a, 0, 0);
-    else                            p0 = new THREE.Vector3(0, 0, -D / b);
+    // foot of perpendicular from origin → keeps the drawn trace near the figure
+    const p0 = new THREE.Vector3(-D * a / denom, 0, -D * b / denom);
     return { p0, dir };
   }
 
@@ -4299,6 +4304,10 @@ class StereoSim {
 
     // Live readout overlay (section type/area/perimeter, last measurement)
     _stereoUpdateReadout(info);
+
+    // Keep the trace-method caption in sync (e.g. right after the 3rd pick),
+    // but only in step mode so the "Кликните 3 точки" instruction is preserved.
+    if (stereoSim && stereoSim._section3PStepBy) _stereoStepHint();
   }
 
   function _stereoUpdateReadout(info) {

frontend/lab.html

@@ -4819,7 +4819,7 @@
 <script src="/js/labs/flask.js"></script>
 <script src="/js/labs/redox.js"></script>
 <script src="/js/labs/ionexchange.js"></script>
-<script src="/js/labs/stereo.js?v=9"></script>
+<script src="/js/labs/stereo.js?v=10"></script>
 <script src="/js/notifications.js"></script>
 <script src="/js/search.js"></script>
 <script src="/js/mobile.js"></script>