Adding a site's address subscribes to the feed it links (#71, #72)

Both add paths, the CLI's and the web's, look behind the URL first: a web
page that names its feed with <link rel="alternate"> is swapped for that
feed, before the duplicate check so it finds a feed someone already has.
Before, the page itself was added and every scan failed on it.

alternate_feed_link found tags in a to_lowercase() copy and sliced the
original at those offsets; Unicode lowercasing changes some characters'
length, so a page with one before its <link> tags lost the href or
panicked off a char boundary. ASCII lowercasing keeps offsets aligned.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/feed.rs

@@ -332,7 +332,9 @@ fn plain_text(bytes: &[u8]) -> Option<String> {
 /// Letters of Note, the Daily Dot, Hell Gate, The Frame Lab and Daily Kos.
 fn alternate_feed_link(bytes: &[u8]) -> Option<String> {
     let text = String::from_utf8_lossy(bytes);
-    let lower = text.to_lowercase();
+    // ASCII only: to_lowercase changes some characters' length (U+0130 grows a byte), and the
+    // offsets found in the lowered copy then sliced the original off a char boundary.
+    let lower = text.to_ascii_lowercase();
     let mut pos = 0;
     while let Some(rel) = lower[pos..].find("<link") {
         let start = pos + rel;
@@ -351,6 +353,22 @@ fn alternate_feed_link(bytes: &[u8]) -> Option<String> {
     None
 }
 
+/// The feed a web page links as its own, for someone who pasted a site's address instead of
+/// its feed's. Anything else, or a page that cannot be read, comes back as given, and adding
+/// it goes on as before.
+pub async fn feed_behind_page(client: &reqwest::Client, url: &str) -> String {
+    let found = async {
+        let resp = client.get(url).timeout(std::time::Duration::from_secs(20)).send().await.ok()?;
+        let base = resp.url().clone();
+        let page = resp.bytes().await.ok()?;
+        if !looks_like_html(&page) {
+            return None;
+        }
+        base.join(&alternate_feed_link(&page)?).ok()
+    };
+    found.await.map(String::from).unwrap_or_else(|| url.to_owned())
+}
+
 /// Artwork for a feed that has none: the icon its site's page names, or else the site's
 /// `/favicon.ico`. None if neither is there.
 pub async fn site_icon(client: &reqwest::Client, site: &str) -> Option<String> {
@@ -1216,6 +1234,14 @@ mod tests {
         assert_eq!(f.entries[2].enclosures.len(), 0, "an item may have none");
     }
 
+    #[test]
+    fn a_feed_link_after_non_ascii_text_is_found() {
+        // U+0130 lowercases to three bytes from two, which once shifted every offset after it.
+        let page = "<html><title>\u{130}stanbul \u{130}\u{130}</title>\
+            <link rel=\"alternate\" type=\"application/rss+xml\" href=\"/feed.xml\">";
+        assert_eq!(alternate_feed_link(page.as_bytes()).as_deref(), Some("/feed.xml"));
+    }
+
     #[test]
     fn page_icon_prefers_the_touch_icon() {
         let page = br#"<head><link rel="stylesheet" href="/a.css">