Fix KeyError in MP4 rendering and require --step for render

Two fixes in the viz command:
1. Use .get() instead of direct dict access in the MP4 frame loop
   to gracefully skip timesteps where the variable is missing,
   preventing a crash mid-render.
2. Require --step argument for rendering instead of silently
   rendering all timesteps, which could fill disk and take very long.

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

Author: sbryngelson

toolchain/mfc/viz/renderer.py

@@ -218,14 +218,17 @@ def render_mp4(varname, steps, output, fps=10,  # pylint: disable=too-many-argum
 
     for i, step in enumerate(step_iter):
         assembled = read_func(step)
+        var_data = assembled.variables.get(varname)
+        if var_data is None:
+            continue
         frame_path = os.path.join(viz_dir, f'{i:06d}.png')
 
         if assembled.ndim == 1:
-            render_1d(assembled.x_cc, assembled.variables[varname],
+            render_1d(assembled.x_cc, var_data,
                       varname, step, frame_path, **opts)
         elif assembled.ndim == 2:
             render_2d(assembled.x_cc, assembled.y_cc,
-                      assembled.variables[varname],
+                      var_data,
                       varname, step, frame_path, **opts)
         elif assembled.ndim == 3:
             render_3d_slice(assembled, varname, step, frame_path, **opts)

toolchain/mfc/viz/viz.py

@@ -134,6 +134,11 @@ def viz():  # pylint: disable=too-many-locals,too-many-statements,too-many-branc
                    "Use --list-vars to see available variables.")
         sys.exit(1)
 
+    if step_arg is None:
+        cons.print("[bold red]Error:[/bold red] --step is required for rendering. "
+                   "Use --list-steps to see available timesteps, or pass --step all.")
+        sys.exit(1)
+
     steps = discover_timesteps(case_dir, fmt)
     if not steps:
         cons.print("[bold red]Error:[/bold red] No timesteps found.")