Fixes #38.

Cause

lib/solid_objects/engine.rb required app/models/solid_objects/record.rb at
the top of the file, and SolidObjects::Record is a subclass of
ActiveRecord::Base. Bundler.require runs that before the host application
boots, so ActiveRecord::Base loaded during railtie setup instead of on first
use.

That moved every deferred on_load(:active_record) hook in the host
application forward. The Active Record railtie registers its encryption
configuration inside on_load(:active_record_encryption). The constant was
already loaded, so that hook ran at once, before config/initializers. An
application that assigns its encryption keys in an initializer got
Encryption.configure with nils, and the later assignment was discarded.

Change

• The engine requires the record class from an ActiveSupport.on_load(:active_record)
  hook, so nothing touches ActiveRecord::Base until the application does.
• The record class applies the configured connects_to in its class body.
  The initializer could no longer call configure_connection: Zeitwerk can
  load record.rb first, and the superclass expression there is what loads
  ActiveRecord::Base, so the hook would run while SolidObjects::Record was
  still undefined. Reading the configuration in the class body also survives a
  development reload, which the initializer did not.

Tests

test/integration/active_record_load_timing_test.rb is new. One test asks a
fresh process whether require "solid_objects" leaves the :active_record
load hooks deferred. The other boots the dummy application, which now assigns
encryption keys in config/initializers, and reads the key back.

test/integration/engine_test.rb gains a test that boots the dummy
application with connects_to configured in an initializer and asserts the
record class owns its own connection pool. That test passes on main, so it
records the behaviour the class-body change had to keep.

Observed failures before the fix:

ActiveRecordLoadTimingTest#test_requiring_the_gem_leaves_the_active_record_load_hooks_deferred
the gem loads ActiveRecord::Base at require time, so host application hooks run too early.
Expected: "deferred"
  Actual: "immediate"

ActiveRecordLoadTimingTest#test_an_application_initializer_configures_Active_Record_encryption
active_record/encryption/config.rb:44: Missing Active Record encryption credential:
active_record_encryption.primary_key (ActiveRecord::Encryption::Errors::Configuration)

With the engine fix in place but configure_connection removed from the record
class body:

EngineTest#test_connects_the_record_class_to_the_configured_database
--- expected
+++ actual
-"ActiveRecord::Base SolidObjects::Record"
+"ActiveRecord::Base"

Validation

bundle exec rake (tests, Standard Ruby, RuboCop, RBS, Steep, Brakeman):
535 runs, 1768 assertions, 0 failures, 0 errors, 15 skips. The skip count
matches main.