gatewayd: strict status lines and chunk lines; no as casts (M4a review, findings 1 and 3)

A status line splits on single spaces only, and every chunk line must end in CRLF. The bounded
`as` casts in http.rs, handshake.rs and proto's sha1.rs become try_from and from.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/proto/src/sha1.rs

@@ -45,7 +45,7 @@ impl Sha1 {
         // Use `split_at` and `get_mut(..)`, no indexing that can go out of bounds.
         self.length = self
             .length
-            .wrapping_add(8u64.wrapping_mul(data.len() as u64));
+            .wrapping_add(8u64.wrapping_mul(u64::try_from(data.len()).unwrap_or(u64::MAX)));
         while !data.is_empty() {
             let space = 64 - self.filled;
             let take = data.len().min(space);