Enhance interactive 3D viz: overlays, server-side rendering, fast playback

Major interactive visualization improvements:

**New features:**
- Contour overlays (2D/3D), isosurface/isovolume mixing
- Solid color isosurfaces, opacity control, improved timestep slider
- Server-side rendering via kaleido on Linux for fast 3D playback over SSH

**Playback performance:**
- Aggressive data prefetch: 3 workers, 15 steps ahead, 40-entry cache
- 3D mesh prefetch: 3 workers, 50K cell budget during playback
- Force Dash patch path during playback to avoid full re-renders
- Prefetch keeps cache warm so playback hits ~0.002s/frame

**Dependencies:**
- Add kaleido for server-side Plotly figure rendering (Linux SSH)
- Remove pyvista (replaced by kaleido)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

toolchain/mfc/viz/_step_cache.py

@@ -28,7 +28,7 @@
 
 logger = logging.getLogger(__name__)
 
-CACHE_MAX: int = 20
+CACHE_MAX: int = 40
 _cache: dict = {}
 _cache_order: list = []
 _in_flight: set = set()   # steps currently being prefetched
@@ -44,7 +44,7 @@ def _get_prefetch_pool() -> ThreadPoolExecutor:
     with _prefetch_pool_lock:
         if _prefetch_pool is None:
             _prefetch_pool = ThreadPoolExecutor(
-                max_workers=1, thread_name_prefix='mfc_prefetch')
+                max_workers=3, thread_name_prefix='mfc_prefetch')
             atexit.register(_prefetch_pool.shutdown, wait=False)
         return _prefetch_pool