fix: QA P1 findings on auto-schema-upgrade (v4.4.3)

Addresses the three P1 findings from QA baseline review of a8a0ce2:

1. uninstall.php: delete psa_db_version option on plugin removal so no
   orphaned data is left behind — satisfies AGENT-OPERATING-STANDARD
   teardown/uninstall DoD.

2. Extract psa_upgrade_needed( $stored, $current ) as a pure function and
   add tests/unit/UpgradeTest.php covering fresh-install, same-version,
   future-version, and older-version scenarios. The impure wrapper
   psa_maybe_run_upgrades() now delegates the decision to the pure helper.

3. Document the psa_db_version option and the admin_init-triggered
   upgrade mechanism in ARCHITECTURE.md (Options section), including
   why admin_init, downgrade behavior, cleanup, and testability.

No behavioral change for end users — same upgrade semantics, just
testable and documented, with proper teardown.

Agent-Session: cowork-qa-p1-fixes-20260414

uninstall.php

@@ -42,6 +42,7 @@ if ( ! is_wp_error( $terms ) && ! empty( $terms ) ) {
 // 3. Delete plugin options.
 delete_option( 'psa_settings' );
 delete_option( 'psa_search_cache_gen' );
+delete_option( 'psa_db_version' );
 
 // 4. Delete all plugin transients (validation cache, rate limits).
 $wpdb->query(