feat(phys8): Phase 0 redesign foundation — CSS + JS infrastructure

Закладывает уникальный визуальный язык и engine'ы для редизайна Физики 8.

CSS:
- phys8-design-system.css (12 КБ): 3 темы (thermal/electric/spectrum),
  тематические hero-палитры, watermarks, animations (thermal-shift,
  electric-pulse, spectrum-drift, wm-breathe/flicker/rotate, noise overlay),
  staggered fade-in для виджетов, soft elevation на карточках,
  monospace для физ. величин, topic-aware progress bars,
  mobile responsive (≤768px), prefers-reduced-motion.
- phys8-interactives.css (10 КБ): .p8-draggable + .p8-droptarget с
  hover-effects, .p8-palette (для circuit-builder), .p8-scrubber,
  .p8-readout табло, .p8-tooltip, .p8-sandbox canvas wrapper,
  .p8-thermometer + .p8-compass-needle SVG-композиции, glow-utility.

JS:
- phys8-anim.js (6 КБ): easing-функции (quad/cubic/expo/back/elastic/
  bounce/spring), tween-engine с onUpdate/onComplete, raf-wrapper,
  oscillate, stagger, onVisible (IntersectionObserver). Экспорт P8Anim.
- phys8-drag.js (12 КБ): универсальный drag-engine. P8Drag.attach()
  для DOM/SVG, P8Drag.attachCanvas() для логических объектов с
  hit-test, P8Drag.attachPalette() для drag-from-palette-to-drop,
  constraints (lockX/Y, bounds, snap-to-grid), touch + mouse + pointer.
- phys8-helpers.js (18 КБ): тематические хелперы. P8Helpers.thermal
  (tempColor 0-1, heatFlowArrow, molecule, thermometerSVG,
  convectionCellParticles), .em (chargeSVG, circuitComponent для
  battery/resistor/lamp/ammeter/voltmeter/switch, fieldLineFrom),
  .optics (rayLine, lensSVG converging/diverging, mirrorPlane),
  .svg utils (el, create, linearGradient, radialGradient,
  gradientArrow, labeledText).

Линковка (redesign_p8_phase0.cjs):
- 2 CSS-link после katex CDN
- 3 JS-link после phys.js/xp.js
- body class p8-theme-thermal/electric/spectrum на ch1/ch2/ch3
- hub и lab — без темы (нейтральный пурпурный brand)

frontend/textbooks/physics_8_lab.html

@@ -8,6 +8,8 @@
 <title>Физика 8 · Лабораторный практикум</title>
 <meta name="viewport" content="width=device-width,initial-scale=1">
 <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/katex@0.16.9/dist/katex.min.css">
+<link rel="stylesheet" href="/css/phys8-interactives.css">
+<link rel="stylesheet" href="/css/phys8-design-system.css">
 <link rel="stylesheet" href="/css/phys-textbook-widgets.css">
 <script defer src="https://cdn.jsdelivr.net/npm/katex@0.16.9/dist/katex.min.js"></script>
 <script defer src="https://cdn.jsdelivr.net/npm/katex@0.16.9/dist/contrib/auto-render.min.js"
@@ -16,6 +18,9 @@
 <script src="/js/xp.js" defer></script>
 <script src="/js/g3d.js" defer></script>
 <script src="/js/phys.js" defer></script>
+<script src="/js/phys8-helpers.js" defer></script>
+<script src="/js/phys8-drag.js" defer></script>
+<script src="/js/phys8-anim.js" defer></script>
 <script src="/js/optics.js" defer></script>
 <link href="https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700;800&family=Manrope:wght@600;700;800;900&family=Unbounded:wght@700;800;900&family=JetBrains+Mono:wght@500;700&display=swap" rel="stylesheet">
 <style>