[0.9.1] FORK_IA_UX: Fase 2 (invite) — invite a contact to a Karvan room by feed id over SSB

Reuses Oasis's own private-message invite pattern (like the industry module:
pmModel.sendMessage([feed], 'KARVAN_INVITE', '... -> /karvan/<id>')), so the invite
lands in the contact's inbox with a link — no new SSB code, no touching the SSB
startup. Joining a room by link adopts a local mirror (karvanModel.adoptRoom, the
id is the capability). Real-time cross-device signaling (media/text sync between
the two mirrors) is the remaining Fase 2 piece (muxrpc ephemeral), deferred.
Verified single-node: invite publishes to the inbox, validation, adopt, form; 13/13
model tests (added adoptRoom).

test/karvan_model.test.js

@@ -137,6 +137,20 @@ async function test(name, fn) {
     assert.ok(m.getRoom(ids[100]));                      // la última sigue
   });
 
+  await test("adoptRoom: espejo local con id dado, idempotente, rechaza id inválido", () => {
+    const m = makeModel();
+    const id = "abc123def456abc789";
+    const r = m.adoptRoom({ id, title: "espejo" });
+    assert.strictEqual(r.id, id);
+    assert.strictEqual(r.title, "espejo");
+    assert.ok(m.getRoom(id));
+    const again = m.adoptRoom({ id, title: "otro" });       // idempotente
+    assert.strictEqual(again.id, id);
+    assert.strictEqual(m.listRooms().filter((x) => x.id === id).length, 1);
+    assert.strictEqual(m.adoptRoom({ id: "NO-hex!" }), null); // id inválido
+    assert.ok(m.postMessage(id, { from: "a", text: "hola" }), "se puede postear en la adoptada");
+  });
+
   console.log("\n" + passed + " passed, " + failed + " failed");
   process.exit(failed ? 1 : 0);
 })();