refactor: consolidate escape functions and extract weight constants

- Create escape.rs with shared html_escape, html_escape_into, xml_escape
- Remove duplicate implementations from render.rs, highlight.rs, feed.rs, sitemap.rs
- Add DEFAULT_WEIGHT (50) and DEFAULT_WEIGHT_HIGH (99) constants to content.rs
- Replace all magic number weight defaults with named constants

No functional changes; all 67 tests pass.

src/escape.rs

@@ -0,0 +1,63 @@
+//! Text escaping utilities for HTML and XML output.
+
+/// Escape HTML special characters for safe embedding in HTML content.
+///
+/// Escapes: `&`, `<`, `>`, `"`
+pub fn html_escape(s: &str) -> String {
+    let mut result = String::with_capacity(s.len());
+    html_escape_into(&mut result, s);
+    result
+}
+
+/// Escape HTML characters into an existing string.
+///
+/// This is more efficient when building output incrementally.
+pub fn html_escape_into(out: &mut String, s: &str) {
+    for c in s.chars() {
+        match c {
+            '&' => out.push_str("&amp;"),
+            '<' => out.push_str("&lt;"),
+            '>' => out.push_str("&gt;"),
+            '"' => out.push_str("&quot;"),
+            _ => out.push(c),
+        }
+    }
+}
+
+/// Escape XML special characters for safe embedding in XML documents.
+///
+/// Escapes: `&`, `<`, `>`, `"`, `'`
+pub fn xml_escape(s: &str) -> String {
+    s.replace('&', "&amp;")
+        .replace('<', "&lt;")
+        .replace('>', "&gt;")
+        .replace('"', "&quot;")
+        .replace('\'', "&apos;")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_html_escape() {
+        assert_eq!(html_escape("Hello & World"), "Hello &amp; World");
+        assert_eq!(html_escape("<tag>"), "&lt;tag&gt;");
+        assert_eq!(html_escape("\"quoted\""), "&quot;quoted&quot;");
+    }
+
+    #[test]
+    fn test_html_escape_into() {
+        let mut buf = String::new();
+        html_escape_into(&mut buf, "a < b");
+        assert_eq!(buf, "a &lt; b");
+    }
+
+    #[test]
+    fn test_xml_escape() {
+        assert_eq!(xml_escape("Hello & World"), "Hello &amp; World");
+        assert_eq!(xml_escape("<tag>"), "&lt;tag&gt;");
+        assert_eq!(xml_escape("\"quoted\""), "&quot;quoted&quot;");
+        assert_eq!(xml_escape("it's"), "it&apos;s");
+    }
+}