Rest a FAB nested in the rail at elevation 0

M3 puts a FAB nested in another component at level 0, and the documented pattern
puts one in the rail's header, so every documented use sat at elevation 3. The
rule is unlayered: the FAB's shadow is a utility and a layer would lose to it.
Plan: docs/plans/material-3-alignment.md, step 14 (navigation N-03).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

resources/css/components/navigation.css

@@ -41,6 +41,9 @@
  * rail in the layout rather than one that opens over a scrim; 75rem (1200px) is `large`, where the
  * rail starts expanded instead of collapsed. The bar's own item layout is a *container* query at
  * the same 37.5rem, so a bar in a narrow column lays out by its own width.
+ *
+ * Everything here is in `@layer components` except the last block, which restyles a FAB the
+ * application put in the rail: that one has to beat a utility, and a layer never does.
  */
 
 @custom-variant rail-collapsed {
@@ -601,3 +604,13 @@
         }
     }
 }
+
+/* A FAB in the rail's header rests at elevation 0, not the 3 a standalone FAB has
+ * (docs/reference/m3/components-navigation-selection-inputs.md § Navigation rail: "when nested
+ * within another component, such as the navigation rail, the FAB's resting elevation should be
+ * level 0"). Unlayered on purpose: the FAB draws its shadow with a utility, and a rule in any
+ * layer loses to a utility — the same reason toolbar.css gives for its vibrant recolouring. */
+[data-navigation-rail-header] [data-fab],
+[data-navigation-rail-header] [data-fab]:hover {
+    box-shadow: none;
+}