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/tests/viewer_tooltip.mjs

@@ -117,5 +117,59 @@ const expect = (actual, expected, msg) => {
   );
 }
 
+// --- #125: doc'd composite + root -------------------------------------------
+// A minimal model whose composite def and root both carry a "doc"; pins that
+// the doc reaches INFO.B (collapsed), INFO.C (expanded), and root.doc.
+//
+// genDot resolves a nested composite reference via the module-scope COMP
+// captured once at require time from window.AURA_MODEL (see the file-top
+// comment above the `model`/`ROOT` constants and the sibling
+// viewer_nested_depth.mjs guard's identical note) — reusing the
+// already-required module's genDot would resolve "sub" back to THIS file's
+// original doc-less "sub" composite of the same name, not docModel's. Fresh-
+// require the module against docModel so its own COMP reflects the doc'd
+// composite.
+const docModel = {
+  root: {
+    nodes: { "0": { comp: "sub" } },
+    edges: [],
+    doc: "root rationale line",
+  },
+  composites: {
+    sub: {
+      inputs: [["f64", "any", "a"]],
+      outputs: [["y", "f64"]],
+      nodes: {},
+      edges: [],
+      doc: "why this sub-graph is wired this way",
+    },
+  },
+};
+const viewerPath = join(here, "..", "assets", "graph-viewer.js");
+delete require.cache[require.resolve(viewerPath)];
+global.window = { AURA_MODEL: docModel };
+const { normalizeModel: normalizeModelDoc, genDot: genDotDoc } = require(viewerPath);
+const docNorm = normalizeModelDoc(docModel);
+expect(docNorm.root.doc, "root rationale line", "root doc carried");
+expect(docNorm.composites.sub.doc, "why this sub-graph is wired this way", "composite doc carried");
+
+{
+  const { info } = genDotDoc(docNorm.root, "root", new Set(), false);
+  expect(
+    info.B["n0"],
+    "**sub** <span class='dim'>· composite</span>\nparams: <span class='dim'>none</span>\nwhy this sub-graph is wired this way",
+    "collapsed tooltip carries the doc paragraph"
+  );
+}
+
+{
+  const { info } = genDotDoc(docNorm.root, "root", new Set(["n0"]), false);
+  expect(
+    info.C["clust_n0"],
+    "**sub** <span class='dim'>· composite · click frame to collapse</span>\nwhy this sub-graph is wired this way",
+    "expanded cluster tooltip carries the doc paragraph"
+  );
+}
+
 console.log("OK — INFO.B/INFO.C/INFO.P tooltip shapes and md() conversion pinned.");
 process.exit(0);