mtglib: promote obfuscation out of internal

Per discussion on #494, this allows external packages (e.g. the upcoming
mtglib/dcprobe for the doctor RPC probe) to reuse the obfuscated2
transport without an internal wrapper.

No public-API change beyond the import path. The only exported names
(Obfuscator, its two methods, and the Secret field) were already
exported within the package.

.mise.toml

 
 [tasks."test:fuzz:client-handshake"]
 description = "Run fuzzy test for ClientHandshake"
-run = "go test -v {{ vars.fuzzflags }} -fuzz=FuzzClientServerHandshake ./mtglib/internal/obfuscation"
+run = "go test -v {{ vars.fuzzflags }} -fuzz=FuzzClientServerHandshake ./mtglib/obfuscation"
 
 [tasks."test:fuzz:server-handshake-frame"]
 description = "Run fuzzy test for GenerateHandshakeFrame"
-run = "go test -v {{ vars.fuzzflags }} -fuzz=FuzzGenerateHandshakeFrame ./mtglib/internal/obfuscation"
+run = "go test -v {{ vars.fuzzflags }} -fuzz=FuzzGenerateHandshakeFrame ./mtglib/obfuscation"
 
 [tasks.static]
 description = "Build static binary"

mtglib/internal/dc/addr.go

 import (
 	"fmt"
 
-	"github.com/9seconds/mtg/v2/mtglib/internal/obfuscation"
+	"github.com/9seconds/mtg/v2/mtglib/obfuscation"
 )
 
 type Addr struct {

mtglib/internal/obfuscation/obfuscator.go → mtglib/obfuscation/obfuscator.go

 	"github.com/9seconds/mtg/v2/essentials"
 )
 
+// Obfuscator implements the obfuscated2 handshake
+// (https://core.telegram.org/mtproto/mtproto-transports#transport-obfuscation).
+// Set Secret to the MTProxy secret for key-mixed handshakes; leave nil for
+// direct DC connections.
 type Obfuscator struct {
 	Secret []byte
 }
 
+// ReadHandshake reads the 64-byte obfuscated2 client handshake from r,
+// validates it, and returns the DC the client requested along with a
+// transparent en/decrypting wrapper over r.
 func (o Obfuscator) ReadHandshake(r essentials.Conn) (int, essentials.Conn, error) {
 	frame := handshakeFrame{}
 
 	return frame.dc(), cn, nil
 }
 
+// SendHandshake writes a fresh 64-byte obfuscated2 handshake for the given
+// DC to w and returns a transparent en/decrypting wrapper over w.
 func (o Obfuscator) SendHandshake(w essentials.Conn, dc int) (essentials.Conn, error) {
 	frame := generateHandshake(dc)
 	copyFrame := frame

mtglib/internal/obfuscation/obfuscator_fuzz_test.go → mtglib/obfuscation/obfuscator_fuzz_test.go

 
 	"github.com/9seconds/mtg/v2/internal/testlib"
 	"github.com/9seconds/mtg/v2/mtglib"
-	"github.com/9seconds/mtg/v2/mtglib/internal/obfuscation"
+	"github.com/9seconds/mtg/v2/mtglib/obfuscation"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/mock"
 )

mtglib/internal/obfuscation/obfuscator_test.go → mtglib/obfuscation/obfuscator_test.go

 
 	"github.com/9seconds/mtg/v2/internal/testlib"
 	"github.com/9seconds/mtg/v2/mtglib"
-	"github.com/9seconds/mtg/v2/mtglib/internal/obfuscation"
+	"github.com/9seconds/mtg/v2/mtglib/obfuscation"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/mock"
 	"github.com/stretchr/testify/require"

mtglib/proxy.go

 	"github.com/9seconds/mtg/v2/essentials"
 	"github.com/9seconds/mtg/v2/mtglib/internal/dc"
 	"github.com/9seconds/mtg/v2/mtglib/internal/doppel"
-	"github.com/9seconds/mtg/v2/mtglib/internal/obfuscation"
+	"github.com/9seconds/mtg/v2/mtglib/obfuscation"
 	"github.com/9seconds/mtg/v2/mtglib/internal/relay"
 	"github.com/9seconds/mtg/v2/mtglib/internal/tls"
 	"github.com/9seconds/mtg/v2/mtglib/internal/tls/fake"