fix: replace String.to_atom with String.to_existing_atom

Prevents BEAM atom table exhaustion from unbounded String.to_atom calls.
Detected by panic-attack assail batch scan (2026-03-30).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

elixir-orchestration/lib/verisim/query/vql_executor.ex

@@ -142,7 +142,7 @@ defmodule VeriSim.Query.VQLExecutor do
     ~w(GRAPH VECTOR TENSOR SEMANTIC DOCUMENT TEMPORAL PROVENANCE SPATIAL)
     |> Enum.filter(&String.contains?(upper, &1))
     |> Enum.map(&String.downcase/1)
-    |> Enum.map(&String.to_atom/1)
+    |> Enum.map(&String.to_existing_atom/1)
   end
 
   # ===========================================================================

elixir-orchestration/lib/verisim/query/vql_type_checker.ex

@@ -308,7 +308,7 @@ defmodule VeriSim.Query.VQLTypeChecker do
   defp normalize_proof_type(_), do: :unknown
 
   defp do_normalize(str) when is_binary(str) do
-    atom = str |> String.downcase() |> String.to_atom()
+    atom = str |> String.downcase() |> String.to_existing_atom()
     if atom in @known_proof_types, do: atom, else: :unknown
   end
   defp do_normalize(atom) when is_atom(atom) do