Bound X-Forwarded-For scanning allocation to the hop cap (closes #133)

forwardedClientAddr now walks the header values in reverse with strings.LastIndexByte instead of joining and splitting, so allocation is bounded by the 64-hop cap rather than by header length: 1.6 MB per call becomes 16 bytes for a 1 MB chain. Semantics are unchanged, verified by differential testing against the previous implementation.

internal/middleware/ratelimit.go

@@ -81,29 +81,45 @@ func (m *Middleware) isTrustedProxy(addr netip.Addr) bool {
 // Only the last maxForwardedHops entries are examined. A longer chain
 // is padding, and running out of hops falls back to the peer address
 // the same way an unreadable hop does.
+//
+// The entries are cut off the right end of each header value in place
+// rather than split out of it: the receiver is unauthenticated and a
+// client can pad the header up to MaxHeaderBytes, so splitting would
+// allocate in proportion to the padding (about 8 MB for a 1 MB
+// header) before the cap could discard any of it. Multiple header
+// values are walked in reverse for the same reason, since joining
+// them copies the whole chain.
 func (m *Middleware) forwardedClientAddr(
 	r *http.Request,
 ) (netip.Addr, bool) {
-	hops := strings.Split(
-		strings.Join(r.Header.Values("X-Forwarded-For"), ","), ",",
-	)
-	if len(hops) > maxForwardedHops {
-		hops = hops[len(hops)-maxForwardedHops:]
-	}
+	seen := 0
 
-	for _, hop := range slices.Backward(hops) {
-		hop = strings.TrimSpace(hop)
-		if hop == "" {
-			continue
-		}
+	for _, value := range slices.Backward(
+		r.Header.Values("X-Forwarded-For"),
+	) {
+		for last := false; !last && seen < maxForwardedHops; seen++ {
+			hop := value
 
-		addr, err := netip.ParseAddr(hop)
-		if err != nil {
-			return netip.Addr{}, false
-		}
+			comma := strings.LastIndexByte(value, ',')
+			if comma < 0 {
+				last = true
+			} else {
+				hop, value = value[comma+1:], value[:comma]
+			}
 
-		if addr = normalizeAddr(addr); !m.isTrustedProxy(addr) {
-			return addr, true
+			hop = strings.TrimSpace(hop)
+			if hop == "" {
+				continue
+			}
+
+			addr, err := netip.ParseAddr(hop)
+			if err != nil {
+				return netip.Addr{}, false
+			}
+
+			if addr = normalizeAddr(addr); !m.isTrustedProxy(addr) {
+				return addr, true
+			}
 		}
 	}