feat: replace Remotion with custom Bradly Engine (Phases 1-3)

- Phase 1: Custom animation engine (interpolate, spring, Easing)
- Phase 2: Custom composition components (AbsoluteFill, Sequence, Img, Video, Audio)
- Phase 3: BradlyPlayer with rAF frame loop, imperative API, controls

Migrated 24 files from remotion/@remotion/player imports to src/engine/.
All type errors from migration resolved. Pre-existing errors remain unchanged.

src/components/composition/CompositionElement.tsx

@@ -1,5 +1,6 @@
 import React, { RefObject, useEffect } from 'react';
-import { Sequence, AbsoluteFill, Img, Video, Audio, interpolate } from 'remotion';
+import { Sequence, AbsoluteFill, Img, Video, Audio } from '../../engine/components';
+import { interpolate } from '../../engine/animation';
 import { TimelineElement, TimelineLayer, DesignMD } from '../../types';
 import { calculateElementTransitions } from './useTransitions';
 import { resolveKeyframes } from './keyframeEngine';