feat(serve): split request read budget into head_timeout + body_timeout

The single request_timeout covered head + body under one wall clock, so a
slow-but-legit body upload (e.g. a trickling cellular IoT client) was
judged by the short head deadline and killed mid-body. Split into:

- head_timeout (default 30s): first byte -> full head parse; the classic
  slowloris surface, kept short.
- body_timeout (default 300s): head parse -> full body; an absolute cap
  sized for slow links, anchored independently once the head has parsed.

read_head no longer returns a shared deadline; run_connection anchors the
body deadline itself. ReadHeadErr::RequestTimeout -> HeadTimeout. Config
and ConnLimits gain body_timeout; request_timeout renamed to head_timeout
(breaking, but this axis is unreleased).

Tests: slow_body_outlives_head_timeout (positive: body survives past the
head clock), fixed_/chunked_body_stall_killed_at_body_timeout (body cap
still bites), slowloris_partial_head_killed_at_head_timeout (head clock
unchanged). 79 lib + 43 integration green; clippy --lib clean.

src/serve.rs

@@ -37,7 +37,14 @@ pub struct Config {
     pub keep_alive_timeout: Duration,
     pub max_header_count:   usize,
     pub read_buf_size:      usize,
-    pub request_timeout:    Duration,
+    /// Wall-clock budget for reading the request HEAD (from first byte to
+    /// full head parse). Kept short — an incomplete head is the classic
+    /// slowloris. See `ConnLimits::head_timeout`.
+    pub head_timeout:       Duration,
+    /// Absolute wall-clock cap on reading the request BODY (from head-parse
+    /// to full body). Sized for slow links, so much larger than
+    /// `head_timeout`. See `ConnLimits::body_timeout`.
+    pub body_timeout:       Duration,
     /// Per-write budget for response bytes (the fixed head+body write, and
     /// each streamed chunk). See `ConnLimits::write_timeout`.
     pub write_timeout:      Duration,
@@ -88,7 +95,8 @@ impl Config {
             keep_alive_timeout: Duration::from_secs(60),
             max_header_count:   64,
             read_buf_size:      8 * 1024,
-            request_timeout:    Duration::from_secs(30),
+            head_timeout:       Duration::from_secs(30),
+            body_timeout:       Duration::from_secs(300),
             write_timeout:      Duration::from_secs(30),
             max_body_bytes:     16 * 1024 * 1024,
             drain_timeout:      Duration::from_secs(30),
@@ -107,7 +115,8 @@ impl Config {
             max_head_bytes:     64 * 1024,
             max_body_bytes:     self.max_body_bytes,
             keep_alive_timeout: self.keep_alive_timeout,
-            request_timeout:    self.request_timeout,
+            head_timeout:       self.head_timeout,
+            body_timeout:       self.body_timeout,
             write_timeout:      self.write_timeout,
             max_frame_payload:  self.max_frame_payload,
             max_message_bytes:  self.max_message_bytes,