feat: add standard nav->content top rhythm (56px)

Content region gets a shared top padding on .page > main so every consuming
site has the same nav-to-content gap (matches newerror's .about spacing).
When newerror adopts the submodule, its per-page-type top paddings become
deltas over this baseline.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01C96hwEU9cM7mJpqATiv7R5

base.css

@@ -97,7 +97,7 @@ a { color: inherit; }
   padding-inline: var(--pad);
 }
 .page { min-height: 100vh; display: flex; flex-direction: column; }
-.page > main { flex: 1 0 auto; }
+.page > main { flex: 1 0 auto; padding-top: 56px; }  /* standard nav->content rhythm */
 
 /* =====================================================================
    Top navigation