fix(fetch): surface semaphore-closed as typed error instead of panic (P1) (#21)

Three call sites in webclaw-fetch used .expect("semaphore closed") on
`Semaphore::acquire()`. Under normal operation they never fire, but
under a shutdown race or adversarial runtime state the spawned task
would panic and be silently dropped from the batch / crawl run — the
caller would see fewer results than URLs with no indication why.

Rewritten to match on the acquire result:
- client::fetch_batch and client::fetch_and_extract_batch_with_options
  now emit BatchResult/BatchExtractResult carrying
  FetchError::Build("semaphore closed before acquire").
- crawler's inner loop emits a failed PageResult with the same error
  string instead of panicking.

Behaviorally a no-op for the happy path. Fixes the silent-dropped-task
class of bug noted in the 2026-04-16 audit.

Version: 0.3.14 -> 0.3.15
CHANGELOG updated.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

crates/webclaw-fetch/src/client.rs

@@ -390,8 +390,14 @@ impl FetchClient {
             let url = url.to_string();
 
             handles.push(tokio::spawn(async move {
-                let _permit = permit.acquire().await.expect("semaphore closed");
-                let result = client.fetch(&url).await;
+                // Don't panic if the semaphore has been closed under us
+                // (adversarial runtime state or shutdown race). Surface a
+                // typed error instead so the caller sees one failed URL in
+                // the batch instead of a silently-dropped task.
+                let result = match permit.acquire().await {
+                    Ok(_permit) => client.fetch(&url).await,
+                    Err(_) => Err(FetchError::Build("semaphore closed before acquire".into())),
+                };
                 (idx, BatchResult { url, result })
             }));
         }
@@ -430,8 +436,10 @@ impl FetchClient {
             let opts = options.clone();
 
             handles.push(tokio::spawn(async move {
-                let _permit = permit.acquire().await.expect("semaphore closed");
-                let result = client.fetch_and_extract_with_options(&url, &opts).await;
+                let result = match permit.acquire().await {
+                    Ok(_permit) => client.fetch_and_extract_with_options(&url, &opts).await,
+                    Err(_) => Err(FetchError::Build("semaphore closed before acquire".into())),
+                };
                 (idx, BatchExtractResult { url, result })
             }));
         }

crates/webclaw-fetch/src/crawler.rs

@@ -294,12 +294,27 @@ impl Crawler {
                 let delay = self.config.delay;
 
                 handles.push(tokio::spawn(async move {
-                    // Acquire permit -- blocks if concurrency limit reached
-                    let _permit = permit.acquire().await.expect("semaphore closed");
-                    tokio::time::sleep(delay).await;
-
+                    // Acquire permit -- blocks if concurrency limit reached.
+                    // Surface semaphore-closed as a failed PageResult rather
+                    // than panicking the spawned task and silently dropping
+                    // it from the batch.
                     let page_start = Instant::now();
-                    let result = client.fetch_and_extract(&url).await;
+                    let result = match permit.acquire().await {
+                        Ok(_permit) => {
+                            tokio::time::sleep(delay).await;
+                            client.fetch_and_extract(&url).await
+                        }
+                        Err(_) => {
+                            warn!(url = %url, depth, "semaphore closed before acquire");
+                            return PageResult {
+                                url,
+                                depth,
+                                extraction: None,
+                                error: Some("semaphore closed before acquire".into()),
+                                elapsed: page_start.elapsed(),
+                            };
+                        }
+                    };
                     let elapsed = page_start.elapsed();
 
                     match result {