fix(material/core): M3 themes no inserting loaded marker

Fixes that all M3 themes were causing a "no theme has been loaded" warning to be logged, because they weren't inserting the loaded marker.

Note: it's tempting to create the marker as a token, but we can't do it because tokens are output under a selector, whereas we want the marker to always be at the top level since we detect it by creating an element and inserting it into the `body`.

Fixes angular#29115.

src/material/core/_core-theme.scss

@@ -16,6 +16,22 @@
 @use './tokens/m2/mat/full-pseudo-checkbox' as tokens-mat-full-pseudo-checkbox;
 @use './tokens/m2/mat/minimal-pseudo-checkbox' as tokens-mat-minimal-pseudo-checkbox;
 
+$_has-inserted-loaded-marker: false;
+
+@mixin _theme-loaded-marker {
+  @if not $_has-inserted-loaded-marker {
+    $_has-inserted-loaded-marker: true !global;
+
+    // Marker that is used to determine whether the user has added a theme to their page.
+    // Needs to be generated at the root, because themes may be nested inside classes.
+    @at-root {
+      .mat-theme-loaded-marker {
+        display: none;
+      }
+    }
+  }
+}
+
 @mixin base($theme) {
   @if inspection.get-theme-version($theme) == 1 {
     @include _theme-from-tokens(inspection.get-theme-tokens($theme, base));
@@ -26,6 +42,9 @@
     @include optgroup-theme.base($theme);
     @include pseudo-checkbox-theme.base($theme);
   }
+
+  // The marker is a concrete style no matter which Material version we're targeting.
+  @include _theme-loaded-marker;
 }
 
 @mixin color($theme) {
@@ -54,14 +73,6 @@
       }
     }
   }
-
-  // TODO(crisbeto): move this into the base.
-  // Marker that is used to determine whether the user has added a theme to their page.
-  @at-root {
-    .mat-theme-loaded-marker {
-      display: none;
-    }
-  }
 }
 
 @mixin typography($theme) {
@@ -127,6 +138,9 @@
       }
     }
   }
+
+  // The marker is a concrete style no matter which Material version we're targeting.
+  @include _theme-loaded-marker;
 }
 
 @mixin _theme-from-tokens($tokens, $options...) {