feat(cli): surface the composite doc in the graph viewer (#125)

The viewer threads the model's doc through normalizeModel into both
composite view states — appended to INFO.B (collapsed box) and INFO.C
(expanded cluster frame) — and shows the root composite's doc as a muted
header line (#rootdoc span in GRAPH_HEAD, empty when absent; the DOM
population sits in the browser-only block, the same boundary as the
breadcrumb). viewer_tooltip.mjs pins the doc'd cases beside the
byte-exact un-doc'd pins; sample-model.json gains the additive doc key;
e2e drives aura graph over a nested-doc blueprint into the emitted page.

closes #125

crates/aura-cli/src/render.rs

@@ -37,6 +37,7 @@ const GRAPH_HEAD: &str = r#"<header>
   <b>aura graph</b>
   <span id="crumb"></span>
   <span class="sub">hover · [+] expand · body drill</span>
+  <span id="rootdoc" class="sub"></span>
 </header>
 <div id="stage"></div>
 <div id="tip"></div>
@@ -261,6 +262,10 @@ mod tests {
         );
         // the chart-only toggle control must not leak onto the graph page
         assert!(!html.contains("xmode-toggle"), "chart toggle leaked onto the graph page");
+        assert!(
+            html.contains(r#"<span id="rootdoc" class="sub"></span>"#),
+            "the graph header carries the (empty) root-doc slot"
+        );
     }
 
     fn sample_chart_data() -> ChartData {