Upgrade PHPStan to 2.x and raise analysis level from 6 to 10

- Bump phpstan/phpstan ^2.0 and szepeviktor/phpstan-wordpress ^2.0
- Move the analysis level into phpstan.neon (single source) and raise it to 10
- Add Val, a runtime coercion helper that narrows untyped WordPress boundary
  values (wpdb rows, REST params, superglobals, options) with explicit checks
  instead of blind casts, plus unit tests
- Type value-object fromRow() params as stdClass (what wpdb returns) and map
  columns through Val so unexpected shapes degrade safely
- Use %i identifier placeholders for table names in all wpdb::prepare() calls
  so every query string is a literal and identifiers are escaped by WordPress;
  raises the minimum WordPress version to 6.2 where %i was introduced
- Guard wpdb::prepare() null result before wpdb::query() in updateTax()
- Fix nullable get_permalink()/strtotime() handling, list types at REST and
  capability call sites, dead null-coalescing on checked superglobals, and
  narrow get_users() results before mapping
- Register Val method names with the ValidatedSanitizedInput sniff so it
  validates the real sanitizer around each superglobal read
- Update repository unit tests for the %i placeholder arguments

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/Auth/LoginPage.php

@@ -3,12 +3,14 @@ declare(strict_types=1);
 
 namespace Unsupervised\Schedular\Auth;
 
+use Unsupervised\Schedular\Val;
+
 class LoginPage {
 
 	/**
 	 * Renders the student login shortcode output.
 	 *
-	 * @param array<string, string> $atts Shortcode attributes (unused — reserved for future options).
+	 * @param array<string> $atts Shortcode attributes (unused — reserved for future options).
 	 */
 	public function render( array $atts ): string { // phpcs:ignore Generic.CodeAnalysis.UnusedFunctionParameter.Found
 		if ( is_user_logged_in() ) {
@@ -26,9 +28,9 @@ class LoginPage {
 
 		if ( isset( $_POST['us_login'] ) && check_admin_referer( 'us_student_login' ) ) {
 			$credentials = [
-				'user_login'    => sanitize_user( wp_unslash( $_POST['log'] ?? '' ) ),
+				'user_login'    => sanitize_user( Val::string( wp_unslash( $_POST['log'] ?? '' ) ) ),
 				// phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotSanitized -- passwords must not be sanitized.
-				'user_password' => wp_unslash( $_POST['pwd'] ?? '' ),
+				'user_password' => Val::string( wp_unslash( $_POST['pwd'] ?? '' ) ),
 				'remember'      => isset( $_POST['rememberme'] ),
 			];