Add --interactive mode: Dash web UI for 3D/2D/1D visualization

New file: toolchain/mfc/viz/interactive.py
- Dark-themed Dash app (Catppuccin Mocha palette)
- Viz modes: Slice (x/y/z + position slider), Isosurface
  (min/max + surface count + caps toggle), Volume
  (opacity + shell count + isomin/isomax); 2D heatmap and 1D line
- Play/Pause with FPS slider and loop toggle via dcc.Interval
- Colormap picker (48 options), log scale, vmin/vmax + auto reset
- Camera angle preserved across updates (uirevision=mode)
- Server-side cache avoids re-reading the same step twice

viz.py: --interactive defaults --step to all; dispatches before PNG/MP4
commands.py: add --interactive / -i and --port flags

Usage:
  ./mfc.sh viz case_dir/ --var pres --interactive
  ./mfc.sh viz case_dir/ --var pres --interactive --port 8080

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

Author: sbryngelson

toolchain/mfc/cli/commands.py

@@ -1017,6 +1017,20 @@
             default=False,
             dest="log_scale",
         ),
+        Argument(
+            name="interactive",
+            short="i",
+            help="Launch an interactive Dash web UI instead of saving PNG/MP4.",
+            action=ArgAction.STORE_TRUE,
+            default=False,
+        ),
+        Argument(
+            name="port",
+            help="Port for the interactive web server (default: 8050).",
+            type=int,
+            default=8050,
+            metavar="PORT",
+        ),
     ],
     examples=[
         Example("./mfc.sh viz case_dir/ --var pres --step 1000", "Plot pressure at step 1000"),
@@ -1031,6 +1045,7 @@
         ("--list-vars", "List available variables"),
         ("--list-steps", "List available timesteps"),
         ("--mp4", "Generate MP4 video"),
+        ("--interactive / -i", "Launch interactive Dash web UI"),
         ("--cmap NAME", "Matplotlib colormap"),
         ("--slice-axis x|y|z", "Axis for 3D slice"),
     ],