perf(encode): emit in place, pool the buffer and cache interface flags

interpres.go

@@ -24,6 +24,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"slices"
 )
 
 // A SyntaxError describes a malformed TOML document, including the 1-based
@@ -423,7 +424,12 @@ func (e *Encoder) MarshalContext(ctx context.Context, v any) ([]byte, error) {
 	enc.ctx = ctx
 	enc.opts = *e
 	if err := enc.encode(v); err != nil {
+		enc.release()
 		return nil, err
 	}
-	return enc.bytes(), nil
+	// The output leaves the pooled buffer as a copy, so the next Marshal
+	// reuses the buffer without touching what the caller holds.
+	out := slices.Clone(enc.buf.Bytes())
+	enc.release()
+	return out, nil
 }