feat(2meet-data-optimizer): throttle shadow_read diffs per key and promote on clean drift checks

- The shadow diff logger records at most 20 rows per entity type + key per hour across all objects, on top of the existing per-object 5-minute window: a key stale on every object (WooCommerce total_sales, written by direct SQL) no longer logs a row per product read.
- The auto-promoter's third condition is now TMDO_Drift_Check::clean_since(): every backfillable group of the entity checked within 2 days (sampled or full) with no drift; no record, no promotion. The shadow diff count no longer gates it (the filter's 4th argument is passed as 0). The Promotion Guard still refuses HivePress / WooCommerce sites.
- Reason classification was dropped (decision 2026-09-27): it needs a schema change and drift-check already reports miss / diff / extra per key.
- Verify: ShadowDiffThrottleTest, AutoPromoterDriftTest (RED before); dev36 simulated user shadow_read: clean_since yes, promotion still refused by the guard, mode unchanged.
- PR: PR-075

includes/diagnostic/class-tmdo-drift-check.php

@@ -114,6 +114,31 @@ final class TMDO_Drift_Check {
 		return $report;
 	}
 
+	/**
+	 * Every backfillable group of the entity was checked within $max_age
+	 * seconds (sampled or full) and none drifted. No record counts as unclean.
+	 *
+	 * @param string $entity_type Entity type.
+	 * @param int    $max_age     Seconds.
+	 */
+	public static function clean_since( string $entity_type, int $max_age ): bool {
+		$status = get_option( self::OPTION, array() );
+		$status = is_array( $status ) ? ( $status[ $entity_type ] ?? array() ) : array();
+		$groups = TMDO_Entity_Migration_Engine::backfillable_groups( $entity_type );
+		if ( empty( $groups ) ) {
+			return false;
+		}
+		foreach ( $groups as $group ) {
+			$last = $status[ $group ] ?? null;
+			if ( ! is_array( $last )
+				|| (int) ( $last['miss'] ?? 0 ) + (int) ( $last['diff'] ?? 0 ) + (int) ( $last['extra'] ?? 0 ) > 0
+				|| (int) strtotime( ( $last['checked_at'] ?? '' ) . ' UTC' ) < time() - $max_age ) {
+				return false;
+			}
+		}
+		return true;
+	}
+
 	/**
 	 * Daily check (wpdo_daily_health_check): sample every entity type written
 	 * to both native and flat (dual_write, shadow_read). In aeav_only flat is