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>

Cargo.lock

@@ -3102,7 +3102,7 @@ dependencies = [
 
 [[package]]
 name = "webclaw-cli"
-version = "0.3.14"
+version = "0.3.15"
 dependencies = [
  "clap",
  "dotenvy",
@@ -3123,7 +3123,7 @@ dependencies = [
 
 [[package]]
 name = "webclaw-core"
-version = "0.3.14"
+version = "0.3.15"
 dependencies = [
  "ego-tree",
  "once_cell",
@@ -3141,7 +3141,7 @@ dependencies = [
 
 [[package]]
 name = "webclaw-fetch"
-version = "0.3.14"
+version = "0.3.15"
 dependencies = [
  "bytes",
  "calamine",
@@ -3163,7 +3163,7 @@ dependencies = [
 
 [[package]]
 name = "webclaw-llm"
-version = "0.3.14"
+version = "0.3.15"
 dependencies = [
  "async-trait",
  "reqwest",
@@ -3176,7 +3176,7 @@ dependencies = [
 
 [[package]]
 name = "webclaw-mcp"
-version = "0.3.14"
+version = "0.3.15"
 dependencies = [
  "dirs",
  "dotenvy",
@@ -3197,7 +3197,7 @@ dependencies = [
 
 [[package]]
 name = "webclaw-pdf"
-version = "0.3.14"
+version = "0.3.15"
 dependencies = [
  "pdf-extract",
  "thiserror",