feat(cli): add --cell-size flag, route png through build_png

crates/bin/src/cli.rs

@@ -65,6 +65,10 @@ struct Cli {
     /// Width in blocks inserted for a space character
     #[arg(long, default_value_t = 4)]
     word_spacing: u32,
+
+    /// Cell size in pixels for PNG export (8-64)
+    #[arg(long, default_value_t = 16, value_parser = clap::value_parser!(u32).range(8..=64))]
+    cell_size: u32,
 }
 
 fn palettes_dir() -> PathBuf {
@@ -160,12 +164,20 @@ pub fn run() -> anyhow::Result<()> {
         .filter(|&name| seen.insert(name))
         .collect();
 
-    // Build and validate in one pass
+    // Build and validate in one pass; route "png" through build_png to honour --cell-size
     let exporters_to_run = format_names
         .iter()
-        .map(|&name| -> anyhow::Result<_> {
-            exporters::build(name, &palette)
-                .ok_or_else(|| anyhow::anyhow!("unknown format '{}'. Available: {}", name, all_names.join(", ")))
+        .map(|&name| -> anyhow::Result<Box<dyn lib::StructureExporter>> {
+            if name == "png" {
+                Ok(exporters::build_png(&palette, cli.cell_size))
+            } else {
+                exporters::build(name, &palette)
+                    .ok_or_else(|| anyhow::anyhow!(
+                        "unknown format '{}'. Available: {}",
+                        name,
+                        all_names.join(", ")
+                    ))
+            }
         })
         .collect::<anyhow::Result<Vec<_>>>()?;