Per-step tests ran in random order against the shared store, which
stops being sound once a step writes. One ordered run against a
private store lets a step that writes change what later steps see,
and keeps those writes away from the store the rest of the suite reads.

Assisted-by: GLM-5.3 via pi

diff --git a/test/pro_tacts/test_macos_exchange.rb b/test/pro_tacts/test_macos_exchange.rb
index 594e97c..4a53c18 100644
--- a/test/pro_tacts/test_macos_exchange.rb
+++ b/test/pro_tacts/test_macos_exchange.rb
@@ -1,14 +1,21 @@
-
 require_relative "../test_helper"
+require_relative "../fixture_data"
+require "pathname"
 require "rack/test"
 
 require_relative "exchange_fixtures"
 require "pro_tacts/web"
 
-# Replays each recorded macOS Contacts request against the app and compares
-# the response, byte for byte, with the recorded baseline. A failure means
+# Replays the recorded macOS Contacts session against the app and compares
+# each response, byte for byte, with the recorded baseline. A failure means
 # the responses drifted from what the working client session saw; either fix
 # the drift or, if the change was deliberate, regenerate with `rake fixtures`.
+#
+# The steps run in sorted order against one freshly seeded store, in a
+# single test: the session is a conversation, and a step that writes has to
+# be able to change what the steps after it see. A failure stops the replay
+# at the first divergent step — the steps after it would be measuring a
+# state the real session never produced.
 class MacosExchangeTest < Minitest::Test
   include Rack::Test::Methods
 
@@ -16,17 +23,28 @@ class MacosExchangeTest < Minitest::Test
     ProTacts::Web
   end
 
-  ExchangeFixtures.steps.each do |step|
-    define_method(:"test_#{step.name}") do
+  def test_the_recorded_session_replays_in_order
+    # A store of its own so the writes the session may make never touch the
+    # one the rest of the suite reads, built fresh so a replay never depends
+    # on what a previous run left behind.
+    directory = Pathname.new(__dir__).parent.parent / "tmp" / "test-exchange"
+    store = FixtureData.install(directory)
+    original = ProTacts::Web.store
+    ProTacts::Web.store = store
+
+    ExchangeFixtures.steps.each do |step|
       request step.path, { method: step.method, input: step.body }.merge(ExchangeFixtures.env_for(step))
 
       expected = ExchangeFixtures.recorded_response(step)
 
-      assert_equal expected.status, last_response.status
+      assert_equal expected.status, last_response.status, "#{step.name}: status"
       expected.headers.each do |name, value|
-        assert_equal value, last_response[name], name
+        assert_equal value, last_response[name], "#{step.name}: #{name}"
       end
-      assert_equal expected.body, last_response.body.chomp
+      assert_equal expected.body, last_response.body.chomp, "#{step.name}: body"
     end
+  ensure
+    ProTacts::Web.store = original
+    store&.close
   end
 end