refactor: pattern-match Aprsme.Encoding helpers

- encoding_info/1 (binary clause): extract build_encoding_info/2 with
  true/false heads on String.valid?/1, so the valid-UTF-8 and invalid
  branches each own their map literal.
- try_utf8_conversion/1: convert_to_utf8/2 dispatches on String.valid?/1
  so the latin1 → UTF-8 fallback is a function head, not an inline if.

No behavior change; all 34 Encoding tests still pass.

lib/aprsme/encoding.ex

@@ -89,25 +89,7 @@ defmodule Aprsme.Encoding do
           invalid_at: non_neg_integer() | nil
         }
   def encoding_info(input) when is_binary(input) do
-    byte_count = byte_size(input)
-
-    if String.valid?(input) do
-      %{
-        valid_utf8: true,
-        byte_count: byte_count,
-        char_count: String.length(input),
-        invalid_at: nil
-      }
-    else
-      invalid_pos = find_invalid_byte_position(input)
-
-      %{
-        valid_utf8: false,
-        byte_count: byte_count,
-        char_count: nil,
-        invalid_at: invalid_pos
-      }
-    end
+    build_encoding_info(String.valid?(input), input)
   end
 
   @spec encoding_info(any()) :: %{
@@ -125,18 +107,35 @@ defmodule Aprsme.Encoding do
     }
   end
 
+  defp build_encoding_info(true, input) do
+    %{
+      valid_utf8: true,
+      byte_count: byte_size(input),
+      char_count: String.length(input),
+      invalid_at: nil
+    }
+  end
+
+  defp build_encoding_info(false, input) do
+    %{
+      valid_utf8: false,
+      byte_count: byte_size(input),
+      char_count: nil,
+      invalid_at: find_invalid_byte_position(input)
+    }
+  end
+
   # Private functions
 
   @spec try_utf8_conversion(binary()) :: binary()
   defp try_utf8_conversion(input) do
-    if String.valid?(input) do
-      input
-    else
-      # Try latin1 to UTF-8 conversion
-      latin1_to_utf8(input)
-    end
+    convert_to_utf8(String.valid?(input), input)
   end
 
+  # Valid UTF-8 already — pass through. Otherwise try latin1 → UTF-8.
+  defp convert_to_utf8(true, input), do: input
+  defp convert_to_utf8(false, input), do: latin1_to_utf8(input)
+
   @spec latin1_to_utf8(binary()) :: binary()
   defp latin1_to_utf8(input) do
     input