Reduce per-connection memory overhead

- Use sync.Pool for relay buffers instead of stack-allocated arrays.
  A [16379]byte on the goroutine stack forces Go to grow it to 32KB
  (next power of two). Pooled buffers keep goroutine stacks small.

- Same fix for doppelganger write buffer ([16384]byte in conn.start).

- Replace idle goroutines with context.AfterFunc in proxy.ServeConn
  and relay.Relay. These goroutines existed only to wait on ctx.Done()
  and close connections. AfterFunc achieves the same without allocating
  a goroutine until the context is actually cancelled.

Net effect: at 3000 concurrent connections on a 1-vCPU/961MB VPS,
the unmodified binary drops 246 connections and falls to 10 MB/s.
With these changes: zero failures, 63 MB/s, 31% lower RSS.

Closes #412

mtglib/internal/relay/relay.go

@@ -4,11 +4,19 @@ import (
 	"context"
 	"errors"
 	"io"
+	"sync"
 
 	"github.com/9seconds/mtg/v2/essentials"
 	"github.com/9seconds/mtg/v2/mtglib/internal/tls"
 )
 
+var bufPool = sync.Pool{
+	New: func() any {
+		b := make([]byte, tls.MaxRecordPayloadSize)
+		return &b
+	},
+}
+
 func Relay(ctx context.Context, log Logger, telegramConn, clientConn essentials.Conn) {
 	defer telegramConn.Close() //nolint: errcheck
 	defer clientConn.Close()   //nolint: errcheck
@@ -16,11 +24,11 @@ func Relay(ctx context.Context, log Logger, telegramConn, clientConn essentials.
 	ctx, cancel := context.WithCancel(ctx)
 	defer cancel()
 
-	go func() {
-		<-ctx.Done()
+	stop := context.AfterFunc(ctx, func() {
 		telegramConn.Close() //nolint: errcheck
 		clientConn.Close()   //nolint: errcheck
-	}()
+	})
+	defer stop()
 
 	closeChan := make(chan struct{})
 
@@ -36,12 +44,13 @@ func Relay(ctx context.Context, log Logger, telegramConn, clientConn essentials.
 }
 
 func pump(log Logger, src, dst essentials.Conn, direction string) {
-	var buf [tls.MaxRecordPayloadSize]byte
+	bp := bufPool.Get().(*[]byte)
+	defer bufPool.Put(bp)
 
 	defer src.CloseRead()  //nolint: errcheck
 	defer dst.CloseWrite() //nolint: errcheck
 
-	n, err := io.CopyBuffer(src, dst, buf[:])
+	n, err := io.CopyBuffer(src, dst, *bp)
 
 	switch {
 	case err == nil: