refactor(handler/forward/remote): split handler into 6 focused files with 41 tests

Split the 339-line handler monolith and 1169-line test blob into 11 files
(5 source + 6 test) following the handler/forward/local/ pattern.

Source files: handler.go (core), forward.go (raw forwarding),
sniffing.go (SnifferBuilder + protocol dispatch), util.go (errors +
recorder + rate limit), metadata.go (config parsing).

Fixes: add sync.Mutex for hop access (race), use sentinel errors for
errors.Is compatibility, add nil-addr guard in checkRateLimit, reuse
SnifferBuilder via Build() instead of per-call construction.

handler/forward/remote/metadata.go

@@ -7,7 +7,7 @@ import (
 	"time"
 
 	"github.com/go-gost/core/bypass"
-	md "github.com/go-gost/core/metadata"
+	mdata "github.com/go-gost/core/metadata"
 	mdutil "github.com/go-gost/x/metadata/util"
 	"github.com/go-gost/x/registry"
 )
@@ -28,7 +28,7 @@ type metadata struct {
 	mitmBypass  bypass.Bypass
 }
 
-func (h *forwardHandler) parseMetadata(md md.Metadata) (err error) {
+func (h *forwardHandler) parseMetadata(md mdata.Metadata) (err error) {
 	h.md.readTimeout = mdutil.GetDuration(md, "readTimeout")
 	if h.md.readTimeout <= 0 {
 		h.md.readTimeout = 15 * time.Second