Make at_exit hook installation opt-in via SimpleCov.start

defaults.rb auto-registered the at_exit block on every require. Move
the registration into a new idempotent SimpleCov.install_at_exit_hook;
SimpleCov.start calls it, SimpleCov.start_tracking deliberately doesn't.
A no-op for normal users — the auto-hook returned early anyway when
start hadn't been called.

Author: sferik

lib/simplecov.rb

@@ -46,6 +46,25 @@ class << self
     def start(profile = nil, &)
       initial_setup(profile, &)
       start_tracking
+      install_at_exit_hook
+    end
+
+    #
+    # Install the at_exit hook that formats results and runs exit-code
+    # checks. `SimpleCov.start` calls this automatically. Idempotent —
+    # safe to call multiple times. Callers that drive the formatting
+    # pipeline themselves (e.g., dogfood test setups) can skip it by
+    # using `start_tracking` directly instead of `start`.
+    #
+    def install_at_exit_hook
+      return if @at_exit_hook_installed
+
+      @at_exit_hook_installed = true
+      Kernel.at_exit do
+        next if SimpleCov.external_at_exit?
+
+        SimpleCov.at_exit_behavior
+      end
     end
 
     #

lib/simplecov/configuration.rb

@@ -216,10 +216,11 @@ def configure(&block)
     #     end
     #
     def at_exit(&block)
-      return proc {} unless active_session? || block
-
       @at_exit = block if block
-      @at_exit ||= proc { SimpleCov.result.format! }
+      return @at_exit if @at_exit
+      return proc {} unless active_session?
+
+      @at_exit = proc { SimpleCov.result.format! }
     end
 
     # Whether SimpleCov has anything to do at exit: the Coverage module

lib/simplecov/defaults.rb

@@ -18,12 +18,6 @@
 # Gotta stash this a-s-a-p, see the CommandGuesser class and i.e. #110 for further info
 SimpleCov::CommandGuesser.original_run_command = "#{$PROGRAM_NAME} #{ARGV.join(' ')}"
 
-at_exit do
-  next if SimpleCov.external_at_exit?
-
-  SimpleCov.at_exit_behavior
-end
-
 # Autoload config from ~/.simplecov if present
 require_relative "load_global_config"