docs(voxel_grain_explorer): halve the volume (N 48 -> 24) for Pyodide

The explorer re-slices voxels + re-cuts three 2D image panels on every plane
drag, which is fine natively but sluggish under Pyodide.  Halving N in each
dimension quarters both the per-frame on-plane voxel count (6912 -> 1728) and
each 2D slice (48² -> 24²), so dragging stays responsive in the browser.

Still above the ~1k voxel threshold, so the WebGPU render path is retained.
Comments updated for the new counts.

Claude-Session: https://claude.ai/code/session_018MLraeaoQBWFDScg72W41h

Author: CSSFrancis

Examples/Interactive/plot_voxel_grain_explorer.py

@@ -29,7 +29,7 @@
 rng = np.random.default_rng(11)
 
 # ── 1. Synthetic 3-D polycrystal: nearest-seed voxel grain map ──────────────
-N = 48                       # volume is N³ voxels, indexed V[z, y, x]
+N = 24                       # volume is N³ voxels, indexed V[z, y, x]
 N_GRAINS = 40
 
 seeds = rng.uniform(0, N, size=(N_GRAINS, 3))            # (z, y, x)
@@ -76,7 +76,8 @@ def random_rotations(n):
 # planes.  This anchors the highlight exactly where the slices intersect,
 # shows real slice contents in 3-D, and scales: the on-plane count is
 # ~3·(N/step)² regardless of N, so it stays fast even for a 256³ volume.
-VSTEP = max(1, N // 48)        # in-plane downsample → ~48² cubes per plane
+VSTEP = max(1, N // 48)        # in-plane downsample, capping at ~48² cubes/plane
+                               # for large N (no downsampling at this N=24)
 
 # Voxel cube size in data units.  A touch larger than VSTEP so the three
 # slabs read as solid sheets rather than a dotted grid.
@@ -140,7 +141,7 @@ def slice_voxels(ix, iy, iz):
     size=VOXSIZE, alpha=0.55,
     x_label="x", y_label="y", z_label="z",
     bounds=((0, N - 1),) * 3, zoom=1.1,
-    # gpu="auto" (default): ~7k cubes is over the ~1k voxel threshold, so the
+    # gpu="auto" (default): ~1.7k cubes is over the ~1k voxel threshold, so the
     # WebGPU instanced path handles each drag re-slice when WebGPU is present.
 )
 v_vol.set_title("Grain volume — drag a plane to re-slice")