First one-shot pass

src/engine/filter.rs

@@ -0,0 +1,20 @@
+pub struct CharFilter {
+    pub allowed: Vec<char>,
+}
+
+impl CharFilter {
+    pub fn new(allowed: Vec<char>) -> Self {
+        Self { allowed }
+    }
+
+    pub fn is_allowed(&self, ch: char) -> bool {
+        self.allowed.contains(&ch) || ch == ' '
+    }
+
+    #[allow(dead_code)]
+    pub fn filter_text(&self, text: &str) -> String {
+        text.chars()
+            .filter(|&ch| self.is_allowed(ch))
+            .collect()
+    }
+}