Add logo rebrand: brand marks, logo page, and brand assets

spec/index.html

@@ -105,6 +105,7 @@
       <article class="instrument">
         <header class="letterhead">
           <p class="doc-class">Company Specification</p>
+          <img class="brand-mark" src="/favicon.svg" width="40" height="40" alt="" />
           <h1>Levkin</h1>
           <p class="doc-tagline">Custom software, automation &amp; AI workflows</p>
           <p class="doc-lead">Master overview of services and engagement terms. This document is not a binding agreement; a statement of work is issued for each engagement.</p>

spec/spec.css

@@ -359,6 +359,14 @@ a:hover { color: var(--link-hover); }
   margin-bottom: 0.75rem;
 }
 
+.brand-mark {
+  display: block;
+  width: 2.5rem;
+  height: 2.5rem;
+  margin: 0 auto 0.85rem;
+  border-radius: 0.55rem;
+}
+
 .letterhead h1 {
   font-size: 1.5rem;
   font-weight: 600;