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 })
             }));
         }