capsite/docs: describe the wiring diagram for screen readers

The SVG gets role="img" with a <title> and a <desc> that says in prose
who refers to what, since the arrows carry the meaning and are invisible
to assistive tech. The rendered PNG is unchanged.

Generated-by: Claude Opus 5.5

docs/cookie-rotation-wiring.html

@@ -9,7 +9,20 @@
   .mono { font-family: "DejaVu Sans Mono", monospace; }
 </style></head>
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-<svg id="c" xmlns="http://www.w3.org/2000/svg" width="1120" height="820" viewBox="0 0 1120 820">
+<svg id="c" xmlns="http://www.w3.org/2000/svg" width="1120" height="820" viewBox="0 0 1120 820"
+  role="img" aria-labelledby="title desc">
+  <title id="title">webapp cookie rotation: who holds what</title>
+  <desc id="desc">Three actors and two makers. Alice holds a reference to a store made by
+makeMapStore, and to diigo-cache, which she wired up. She rotates the cookie
+by saying: await E(store).set("cookie", freshCookie). The store, and its
+read-only facets, both refer to the store's kv table, whose one row maps
+"cookie" to the session; its shapes are keyShape "cookie" and valueShape
+M.string(). makeWebApp makes access objects; each refers to its init
+table: url, headers, and headerStore. headerStore refers to the store's
+read-only facet, which access reads at every send. diigo-cache holds a
+reference to access. Mallory, who learned the instance id, also holds a
+reference to access, so she can use the cookie, but she has no reference
+to the store, so she cannot rotate it or swap in her own session.</desc>
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