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

lib/phronesis/cli.ex

@@ -897,7 +897,7 @@ defmodule Phronesis.CLI do
   end
 
   defp run_benchmark(name, opts) do
-    benchmark = String.to_atom(name)
+    benchmark = String.to_existing_atom(name)
     iterations = Keyword.get(opts, :iterations, 10_000)
 
     result = Phronesis.Benchmark.run(benchmark, iterations: iterations)

lib/phronesis/compiler.ex

@@ -619,7 +619,7 @@ defmodule Phronesis.Compiler do
 
     value =
       case base do
-        %{} = map -> Map.get(map, String.to_atom(field)) || Map.get(map, field)
+        %{} = map -> Map.get(map, String.to_existing_atom(field)) || Map.get(map, field)
         _ -> nil
       end
 
@@ -632,7 +632,7 @@ defmodule Phronesis.Compiler do
     value =
       case base do
         nil -> nil
-        %{} = map -> Map.get(map, String.to_atom(field)) || Map.get(map, field)
+        %{} = map -> Map.get(map, String.to_existing_atom(field)) || Map.get(map, field)
         _ -> nil
       end
 

lib/phronesis/consensus/raft/rpc.ex

@@ -108,7 +108,7 @@ defmodule Phronesis.Consensus.Raft.RPC do
   defp parse_distributed_node(node_id) do
     case String.split(node_id, "@") do
       [name, host] ->
-        node = String.to_atom("#{name}@#{host}")
+        node = String.to_existing_atom("#{name}@#{host}")
 
         if node in [node() | Node.list()] do
           {:ok, node, {:via, Registry, {Phronesis.Consensus.Registry, name}}}

lib/phronesis/consensus/supervisor.ex

@@ -19,7 +19,7 @@ defmodule Phronesis.Consensus.Supervisor do
   def init(_opts) do
     # Check if consensus mode is enabled via environment
     consensus_enabled = System.get_env("PHRONESIS_CONSENSUS_ENABLED", "false") == "true"
-    node_id = String.to_atom(System.get_env("PHRONESIS_NODE_ID", "node1"))
+    node_id = String.to_existing_atom(System.get_env("PHRONESIS_NODE_ID", "node1"))
 
     children =
       if consensus_enabled do

lib/phronesis/interpreter.ex

@@ -375,7 +375,7 @@ defmodule Phronesis.Interpreter do
 
   defp resolve_module(["Std", module_name | _rest] = path) do
     # Map Std.BGP → Phronesis.Stdlib.BGP, Std.RPKI → Phronesis.Stdlib.RPKI, etc.
-    elixir_module = Module.concat([Phronesis, Stdlib, String.to_atom(module_name)])
+    elixir_module = Module.concat([Phronesis, Stdlib, String.to_existing_atom(module_name)])
 
     case Code.ensure_loaded(elixir_module) do
       {:module, _mod} ->