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/render.rs

@@ -1,5 +1,6 @@
 //! Markdown to HTML rendering via pulldown-cmark with syntax highlighting.
 
+use crate::escape::html_escape;
 use crate::highlight::{highlight_code, Language};
 use pulldown_cmark::{CodeBlockKind, Event, HeadingLevel, Options, Parser, Tag, TagEnd};
 use serde::Serialize;
@@ -247,13 +248,6 @@ pub fn markdown_to_html(markdown: &str) -> (String, Vec<Anchor>) {
     (html_output, anchors)
 }
 
-fn html_escape(s: &str) -> String {
-    s.replace('&', "&amp;")
-        .replace('<', "&lt;")
-        .replace('>', "&gt;")
-        .replace('"', "&quot;")
-}
-
 /// Convert heading text to a URL-friendly slug ID.
 fn slugify(text: &str) -> String {
     text.to_lowercase()