fix(bench): align the architecture switch with the content column

The switch is inserted as a sibling of `<main class="container">`, so it
sat in the body with no width constraint and rendered flush against the
viewport edge while everything below it was centred in a 920px column.

It now carries `container` as well. Its own margin has to name the auto
sides itself: `.container` is declared earlier in the sheet, so at equal
specificity the `.arch-switch` margin wins and a plain `0 0 0.5rem`
would have left it flush again.

Author: darmie

website/benchmark/index.html

@@ -155,10 +155,13 @@
 
     /* Architecture switch. Shown only when more than one was
        published, so a single-architecture run looks as it always did. */
+    /* Carries `.container` too, so it lines up with the column rather
+       than the viewport. `.container` is declared above, so this margin
+       wins at equal specificity and has to keep the auto sides itself. */
     .arch-switch {
       display: flex;
       gap: 0.35rem;
-      margin: 0 0 0.5rem;
+      margin: 0 auto 0.5rem;
       flex-wrap: wrap;
     }
     .arch-switch button {
@@ -880,7 +883,7 @@ <h2>${escape(title)}</h2>
 
       if (datasets.length > 1) {
         const bar = document.createElement('div');
-        bar.className = 'arch-switch';
+        bar.className = 'arch-switch container';
         datasets.forEach((d, i) => {
           const b = document.createElement('button');
           b.type = 'button';