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/pendulum.js

@@ -1,4 +1,4 @@
-'use strict';
+'use strict';
 /* ══════════════════════════════════════════════════════════════
    PendulumSim — simple pendulum simulation
    θ'' = -(g/L)sin(θ) − γ·θ'
@@ -402,3 +402,51 @@ class PendulumSim {
     });
   }
 }
+
+/* ─── lab UI init ─────────────────────────────────── */
+  var pendSim = null;
+
+  function _openPendulum() {
+    document.getElementById('sim-topbar-title').textContent = 'Маятник';
+    _simShow('sim-pendulum');
+    _registerSimState('pendulum', () => pendSim?.getParams(), st => pendSim?.setParams(st));
+    if (_embedMode) _startStateEmit('pendulum');
+    requestAnimationFrame(() => requestAnimationFrame(() => {
+      if (!pendSim) {
+        pendSim = new PendulumSim(document.getElementById('pendulum-canvas'));
+        pendSim.onUpdate = _pendUpdateUI;
+      }
+      pendSim.fit();
+      pendSim.play();
+    }));
+  }
+
+  function pendParam(name, val) {
+    const v = parseFloat(val);
+    const ids = { theta: 'pend-theta-val', L: 'pend-L-val', g: 'pend-g-val', damping: 'pend-damp-val' };
+    const el = document.getElementById(ids[name]);
+    if (el) el.textContent = v % 1 === 0 ? v : v.toFixed(name === 'g' ? 2 : 1);
+    if (pendSim) pendSim.setParams({ [name]: v });
+  }
+
+  function pendPreset(theta, L, g, damp) {
+    document.getElementById('sl-pend-theta').value = theta; document.getElementById('pend-theta-val').textContent = theta;
+    document.getElementById('sl-pend-L').value = L;         document.getElementById('pend-L-val').textContent = L;
+    document.getElementById('sl-pend-g').value = g;         document.getElementById('pend-g-val').textContent = g;
+    document.getElementById('sl-pend-damp').value = damp;   document.getElementById('pend-damp-val').textContent = damp;
+    if (pendSim) {
+      pendSim.setParams({ theta, L, g, damping: damp });
+      pendSim.play();
+    }
+  }
+
+  function _pendUpdateUI(info) {
+    const v = (id, val) => { const el = document.getElementById(id); if (el) el.textContent = val; };
+    v('pendbar-v1', info.angle);
+    v('pendbar-v2', info.omega);
+    v('pendbar-v3', info.period);
+    v('pendbar-v4', info.energy);
+  }
+
+  /* ── equilibrium ── */
+