fix(crawler): wait for page marker instead of fixed 1s sleep (0.36.2)

A chromium snapshot taken between the wrapper-render and row-render
phases let parse_chapter_list return Ok(vec![]) for a manga that
actually has chapters — the soft-drop branch in sync_manga_chapters
then flipped every existing chapter to dropped_at.

Add wait_for_selector to crawler::nav. navigate() now takes a CSS
marker matching the most-specific element the downstream parser will
look for (one of LIST_PAGE_MARKER / DETAIL_PAGE_CHAPTERS_MARKER /
DETAIL_PAGE_LAYOUT_MARKER). The wait is best-effort and capped by
SELECTOR_TIMEOUT (10s); a legitimately empty page can still pass
through because the parser's #chapter_table sentinel and the
universal broken-page body check stay in force.

Same pattern wired at the reader nav (a#pic_container) and probe
nav (#logo), replacing the implicit assumption that the post-load
JS had finished within 1 second.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/crawler/nav.rs

@@ -43,6 +43,48 @@ pub async fn wait_for_nav(page: &Page) -> Result<(), NavError> {
     }
 }
 
+/// Poll interval for [`wait_for_selector`]. 100ms is fast enough that a
+/// page rendering in 200ms isn't held back noticeably, and slow enough
+/// not to spam CDP with `find_element` calls on a page that's actually
+/// taking its time.
+const SELECTOR_POLL_INTERVAL: Duration = Duration::from_millis(100);
+
+/// Wait until `selector` matches at least one element on `page`, or
+/// `timeout` elapses. Used after a navigation to confirm a page-type-
+/// specific marker is in the DOM before parsing — replaces the fixed
+/// post-nav sleep that previously masked partial-render races.
+///
+/// chromiumoxide 0.7.0 has no built-in `wait_for_selector`, so we poll
+/// `find_element` at [`SELECTOR_POLL_INTERVAL`] until success or budget
+/// exhaustion. A failed `find_element` is *not* an error here — it just
+/// means "not yet" — we only surface an error once the overall
+/// `timeout` is up.
+pub async fn wait_for_selector(
+    page: &Page,
+    selector: &str,
+    timeout: Duration,
+) -> Result<(), NavError> {
+    let deadline = tokio::time::Instant::now() + timeout;
+    loop {
+        if page.find_element(selector).await.is_ok() {
+            return Ok(());
+        }
+        if tokio::time::Instant::now() >= deadline {
+            return Err(NavError::Timeout(timeout));
+        }
+        let remaining = deadline.saturating_duration_since(tokio::time::Instant::now());
+        let sleep_for = SELECTOR_POLL_INTERVAL.min(remaining);
+        tokio::time::sleep(sleep_for).await;
+    }
+}
+
+/// Per-page-type budget for [`wait_for_selector`]. Shorter than
+/// [`NAV_TIMEOUT`] because by the time we're waiting on a selector, the
+/// page has already responded — we're only absorbing post-load JS
+/// finishing its row injection, which on a healthy site takes well
+/// under a second.
+pub const SELECTOR_TIMEOUT: Duration = Duration::from_secs(10);
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -65,4 +107,28 @@ mod tests {
         let e = NavError::Timeout(Duration::from_secs(30));
         assert_eq!(e.to_string(), "navigation timed out after 30s");
     }
+
+    /// Same sanity check as [`timeout_elapses_on_a_future_that_never_resolves`],
+    /// but for the [`wait_for_selector`] polling pattern: the loop must
+    /// surrender on `Elapsed` rather than spinning past the deadline.
+    #[tokio::test(flavor = "current_thread", start_paused = true)]
+    async fn selector_polling_pattern_surrenders_at_deadline() {
+        let timeout = Duration::from_millis(300);
+        let start = tokio::time::Instant::now();
+        let deadline = start + timeout;
+        // Simulate find_element forever returning "not found".
+        let mut polls = 0u32;
+        let result: Result<(), NavError> = loop {
+            polls += 1;
+            if tokio::time::Instant::now() >= deadline {
+                break Err(NavError::Timeout(timeout));
+            }
+            tokio::time::sleep(SELECTOR_POLL_INTERVAL).await;
+        };
+        assert!(matches!(result, Err(NavError::Timeout(_))));
+        // 300ms / 100ms poll interval ≈ 3 iterations plus the final check
+        // that breaks out. Allow some slack since the first poll happens
+        // before any sleep.
+        assert!(polls >= 3, "expected at least 3 poll iterations, got {polls}");
+    }
 }