fix: prevent Config DEFAULTS mutation through deep_merge

Freeze inner hashes in DEFAULTS and rewrite deep_merge to build fresh
hashes with {}.merge for uncopied inner maps. Previously, inner hashes
shared objects with the frozen-looking DEFAULTS, so mutating a loaded
config silently poisoned all later loads.

test/config_test.rb

@@ -27,6 +27,14 @@ class ConfigTest < Minitest::Test
     assert_equal "0.0.0.0", Volumen::Config.load(path: MISSING).host
   end
 
+  def test_does_not_mutate_default_inner_hashes
+    config = Volumen::Config.load(path: MISSING)
+    config.site["title"] = "MUTATED"
+    fresh = Volumen::Config.load(path: MISSING)
+    assert_equal "My Blog", fresh.site["title"],
+                 "mutating a loaded config must not poison DEFAULTS for later loads"
+  end
+
   def test_loads_and_merges_file
     Dir.mktmpdir do |dir|
       path = File.join(dir, "config.toml")