feat(2meet-data-optimizer): flag new Hot zone registrations as deprecated

- ADR-003 step 4 (core side): TMDO_Schema_Registry::register() raises
  _doing_it_wrong() the first time each provider registers a zone=hot
  field in a request (zone omitted defaults to hot). Fields still
  register, so existing hot data stays readable.
- Cold / warm / archive registrations are unaffected.
- Test bootstrap's _doing_it_wrong stub now records calls.
- Verify: core unit 658 / integration 459, HP 142/49, WC 10/22 green,
  phpcs clean
- PR: PR-038

includes/class-tmdo-schema-registry.php

@@ -67,6 +67,13 @@ class TMDO_Schema_Registry {
 	 */
 	private array $warm_fields = array();
 
+	/**
+	 * Providers already told the Hot zone is frozen (once per request).
+	 *
+	 * @var array<string, true>
+	 */
+	private array $hot_deprecation_flagged = array();
+
 	/**
 	 * Private constructor — use instance() to get the singleton.
 	 */
@@ -138,6 +145,7 @@ class TMDO_Schema_Registry {
 		switch ( $config['zone'] ) {
 			case 'hot':
 				$this->hot_columns[ $config['post_type'] ][ $config['column'] ] = $config['data_type'];
+				$this->flag_hot_deprecation( $provider, $config['meta_key'] );
 				break;
 
 			case 'warm':
@@ -159,6 +167,29 @@ class TMDO_Schema_Registry {
 		}
 	}
 
+	/**
+	 * The Hot zone is frozen (ADR-003): fields still register so existing hot
+	 * data stays readable, but each provider gets one notice per request.
+	 *
+	 * @param string $provider Provider name.
+	 * @param string $meta_key First hot meta_key seen from it.
+	 */
+	private function flag_hot_deprecation( string $provider, string $meta_key ): void {
+		if ( isset( $this->hot_deprecation_flagged[ $provider ] ) ) {
+			return;
+		}
+		$this->hot_deprecation_flagged[ $provider ] = true;
+		_doing_it_wrong(
+			__CLASS__ . '::register',
+			sprintf(
+				'Provider "%1$s" registered Hot zone field "%2$s". The Hot zone is frozen; register an entity group (TMDO_Entity_Registry::register_group) instead.',
+				esc_html( $provider ),
+				esc_html( $meta_key )
+			),
+			esc_html( TMDO_VERSION )
+		);
+	}
+
 	/**
 	 * Bulk register multiple fields.
 	 *