refactor: AprsSymbol.normalize_symbol_table pattern-matches on input

- Three-way cond (known-table / overlay-capable char / fallback) becomes
  three function heads: a guard-matched clause for the canonical
  "/"/"\\"/"]" tables, an is_binary clause that decides via regex match
  whether the input is a single alphanumeric overlay character, and a
  catch-all fallback.

lib/aprsme_web/aprs_symbol.ex

@@ -164,14 +164,19 @@ defmodule AprsmeWeb.AprsSymbol do
       "/"
   """
   @spec normalize_symbol_table(String.t() | nil) :: String.t()
-  def normalize_symbol_table(symbol_table) do
-    cond do
-      symbol_table in ["/", "\\", "]"] -> symbol_table
-      symbol_table && String.match?(symbol_table, ~r/^[A-Z0-9]$/) -> "]"
-      true -> "/"
+  def normalize_symbol_table(table) when table in ["/", "\\", "]"], do: table
+
+  def normalize_symbol_table(table) when is_binary(table) do
+    # A single alphanumeric character is an overlay — map to the overlay table.
+    if String.match?(table, ~r/^[A-Z0-9]$/) do
+      "]"
+    else
+      "/"
     end
   end
 
+  def normalize_symbol_table(_), do: "/"
+
   @doc """
   Normalizes a symbol code.