Add PHPUnit baseline test suite

[sultann]

Adds a PHPUnit baseline test suite for Serial Numbers (free), characterizing current behavior before the b8 framework migration so the suite stays green through the port.

Harness

• tests/bootstrap.php loads WooCommerce + the plugin (keeps CPT order storage)
• tests/bin/install.sh provisions the WP + WooCommerce test environment
• composer test:setup then composer test

Coverage (8 files, 42 tests)

• EncryptionTest, KeyModelTest, OrderFulfillmentTest
• RestApiValidateTest, RestApiActivateTest
• StockTest, CronExpiryTest, InstallerTest

Risk-targeted: encryption round-trips, key model + encrypted lookup, order fulfillment lifecycle (assign/revoke/reuse/refund), the wcsn REST validate/activate/deactivate contract and permission gate, checkout stock guard, expiry cron, and installer state.

Note

The wcsn REST routes declare an invalid email schema type ('email'), which WordPress flags as doing_it_wrong; the baseline pins this. Worth fixing to type 'string' / format 'email'.

Run: composer install && composer test:setup && composer test