refactor(handler/http): split handler monolith into 7 focused files with 96 unit tests

Extract the 1024-line handler.go into concern-focused files:
  util.go       — 5 pure functions (decodeServerName, basicProxyAuth, upgradeType,
                  normalizeHostPort, buildConnectResponse) — 100% coverage
  auth.go       — authenticate (5 probe-resistance strategies + knock) + checkRateLimit
  connect.go    — handleConnect, sniffAndHandle, dial, setupTrafficLimiter
  proxy.go      — handleProxy keep-alive loop, proxyRoundTrip, handleUpgradeResponse
  websocket.go  — WebSocket frame sniffing/recording with rate-limited sampling
  udp.go        — UDP-over-HTTP relay with SOCKS5 tunnel
  metadata.go   — metadata struct, parseMetadata, probeResistance type

Key improvements:
- Transport injection (http.RoundTripper field) enables testing without real network
- Nil Router guards in dial, handleUDP, and nil Addr guard in checkRateLimit
- Pure functions converted from methods to package-level (no state dependency)
- setupTrafficLimiter returns cleanup closure to preserve defer lifetime
- handleConnect accepts caller's resp for correct recorder status capture
- Comprehensive package doc with full request-flow documentation

96 tests: 100% on pure functions, 86% on metadata parsing, 55% overall (integration
paths verified by e2e tests).

handler/http/util.go

@@ -0,0 +1,86 @@
+package http
+
+import (
+	"encoding/base64"
+	"encoding/binary"
+	"errors"
+	"hash/crc32"
+	"net"
+	"net/http"
+	"strings"
+
+	"golang.org/x/net/http/httpguts"
+)
+
+// decodeServerName decodes a GOST v2 Gost-Target / X-Gost-Target header
+// value. The encoding is: base64(CRC32(hostname) + base64(hostname)).
+// It verifies the CRC32 checksum before returning the decoded hostname.
+func decodeServerName(s string) (string, error) {
+	b, err := base64.RawURLEncoding.DecodeString(s)
+	if err != nil {
+		return "", err
+	}
+	if len(b) < 4 {
+		return "", errors.New("invalid name")
+	}
+	v, err := base64.RawURLEncoding.DecodeString(string(b[4:]))
+	if err != nil {
+		return "", err
+	}
+	if crc32.ChecksumIEEE(v) != binary.BigEndian.Uint32(b[:4]) {
+		return "", errors.New("invalid name")
+	}
+	return string(v), nil
+}
+
+// basicProxyAuth extracts the username and password from an HTTP Basic
+// authentication Proxy-Authorization header value. It returns ok=false
+// if the value is empty, not a Basic scheme, or not valid base64.
+func basicProxyAuth(proxyAuth string) (username, password string, ok bool) {
+	if proxyAuth == "" {
+		return
+	}
+
+	if !strings.HasPrefix(proxyAuth, "Basic ") {
+		return
+	}
+	c, err := base64.StdEncoding.DecodeString(strings.TrimPrefix(proxyAuth, "Basic "))
+	if err != nil {
+		return
+	}
+	cs := string(c)
+	username, password, ok = strings.Cut(cs, ":")
+	if !ok {
+		return
+	}
+	return username, password, true
+}
+
+// upgradeType returns the upgrade protocol token from an HTTP header.
+// It returns "" if the Connection header does not include an "Upgrade"
+// token or if the Upgrade header is absent.
+func upgradeType(h http.Header) string {
+	if !httpguts.HeaderValuesContainsToken(h["Connection"], "Upgrade") {
+		return ""
+	}
+	return h.Get("Upgrade")
+}
+
+// normalizeHostPort ensures a host string includes a port. If the host
+// already contains a port, it is returned unchanged. Otherwise the
+// defaultPort is appended. IPv6 addresses are correctly handled: bare
+// addresses get bracketed by net.JoinHostPort, and bracketed addresses
+// are stripped of brackets before re-joining.
+func normalizeHostPort(host, defaultPort string) string {
+	if _, port, _ := net.SplitHostPort(host); port == "" {
+		return net.JoinHostPort(strings.Trim(host, "[]"), defaultPort)
+	}
+	return host
+}
+
+// buildConnectResponse returns the raw bytes of an HTTP 200 Connection
+// established response with the given Proxy-Agent header.
+func buildConnectResponse(proxyAgent string) []byte {
+	return []byte("HTTP/1.1 200 Connection established\r\n" +
+		"Proxy-Agent: " + proxyAgent + "\r\n\r\n")
+}