refactor: distribute lab-init.js into 34 engine files

lab-init.js: 4098 -> 543 lines (infrastructure + THEORY only)

Each sim's _open*() + UI helpers moved to its engine file:
graph.js, projectile.js, collision.js, magnetic.js, triangle.js,
geometry.js, trigcircle.js, gas.js (molphys), coulomb.js, circuit.js,
reactions.js (chemistry), newton.js (dynamics), chemsandbox.js,
celldivision.js, photosynthesis.js, angrybirds.js, quadratic.js,
normaldist.js, graphtransform.js, pendulum.js, equilibrium.js,
thinlens.js, mirror.js, isoprocess.js, titration.js, refraction.js,
probability.js, bohratom.js, electrolysis.js, waves.js,
crystal.js, orbitals.js, stereo.js, hydrostatics.js

All 34 engine files syntax-checked OK.

frontend/js/labs/normaldist.js

@@ -1,4 +1,4 @@
-'use strict';
+'use strict';
 /**
  * NormalDistSim v2 — интерактивное нормальное распределение
  * μ, σ · правило 68-95-99.7 · Z-score · закрашивание области
@@ -392,3 +392,51 @@ class NormalDistSim {
     cv.addEventListener('touchend', () => { this.hx = null; this.draw(); });
   }
 }
+
+/* ─── lab UI init ─────────────────────────────────── */
+  var ndSim = null;
+
+  function _openNormalDist() {
+    document.getElementById('sim-topbar-title').textContent = 'Нормальное распределение';
+    _simShow('sim-normaldist');
+    _registerSimState('normaldist', () => ndSim?.getParams(), st => ndSim?.setParams(st));
+    if (_embedMode) _startStateEmit('normaldist');
+    requestAnimationFrame(() => requestAnimationFrame(() => {
+      if (!ndSim) {
+        ndSim = new NormalDistSim(document.getElementById('normaldist-canvas'));
+        ndSim.onUpdate = _ndUpdateUI;
+      }
+      ndSim.fit();
+      ndSim.draw();
+      ndSim._emit();
+    }));
+  }
+
+  function ndParam(name, val) {
+    const v = parseFloat(val);
+    const elId = name === 'mu' ? 'nd-mu-val' : 'nd-sigma-val';
+    document.getElementById(elId).textContent = v % 1 === 0 ? v : v.toFixed(1);
+    if (ndSim) ndSim.setParams({ [name]: v });
+  }
+
+  function ndShade(mode, btn) {
+    document.querySelectorAll('.nd-shade-btn').forEach(b => b.classList.remove('active'));
+    if (btn) btn.classList.add('active');
+    if (ndSim) ndSim.setParams({ shade: mode });
+  }
+
+  function ndPreset(mu, sigma) {
+    document.getElementById('sl-nd-mu').value = mu;     document.getElementById('nd-mu-val').textContent = mu;
+    document.getElementById('sl-nd-sigma').value = sigma; document.getElementById('nd-sigma-val').textContent = sigma;
+    if (ndSim) ndSim.setParams({ mu, sigma });
+  }
+
+  function _ndUpdateUI(info) {
+    const v = (id, val) => { const el = document.getElementById(id); if (el) el.textContent = val; };
+    v('ndbar-v1', info.mu);
+    v('ndbar-v2', info.sigma);
+    v('ndbar-v3', info.peak);
+    v('ndbar-v4', info.area);
+  }
+
+  /* ── graph transform ── */