Implement pagination algorithm with TDD

Add calculate_pages function that splits text into page boundaries
by character offset. Includes tests for empty, single-page, multi-page,
and zero-chars-per-page edge cases.

src/reader.rs

@@ -0,0 +1,64 @@
+pub fn calculate_pages(text: &str, chars_per_page: usize) -> Vec<usize> {
+    let mut pages = Vec::new();
+    pages.push(0);
+
+    if text.is_empty() || chars_per_page == 0 {
+        return pages;
+    }
+
+    let total_chars = text.chars().count();
+    if total_chars <= chars_per_page {
+        pages.push(total_chars);
+        return pages;
+    }
+
+    let mut pos = 0;
+    while pos < total_chars {
+        pos = (pos + chars_per_page).min(total_chars);
+        pages.push(pos);
+    }
+
+    pages
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_pagination_empty() {
+        let pages = calculate_pages("", 100);
+        assert_eq!(pages, vec![0]);
+    }
+
+    #[test]
+    fn test_pagination_shorter_than_page() {
+        let pages = calculate_pages("Hello World", 100);
+        assert_eq!(pages, vec![0, 11]);
+    }
+
+    #[test]
+    fn test_pagination_exact_fit() {
+        let pages = calculate_pages("ABCD", 4);
+        assert_eq!(pages, vec![0, 4]);
+    }
+
+    #[test]
+    fn test_pagination_multiple_pages() {
+        let text = "A".repeat(100);
+        let pages = calculate_pages(&text, 30);
+        assert_eq!(pages, vec![0, 30, 60, 90, 100]);
+    }
+
+    #[test]
+    fn test_pagination_single_char() {
+        let pages = calculate_pages("A", 1);
+        assert_eq!(pages, vec![0, 1]);
+    }
+
+    #[test]
+    fn test_pagination_zero_chars_per_page() {
+        let pages = calculate_pages("test", 0);
+        assert_eq!(pages, vec![0]);
+    }
+}