feat(simulation): add NavalDomain::serializeStateJson for WASM state export

Returns {"t":...,"vessels":[{"id":...,"x":...,"psi":...,"u":...,"r":...},...]}
2 tests verify JSON structure and time accumulation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

libs/simulation/include/ymir/simulation/NavalDomain.h

@@ -87,6 +87,14 @@ public:
     /** Return Euclidean distance between two registered bodies. Asserts both ids exist. */
     double distanceBetween(int idA, int idB) const;
 
+    /**
+     * Serialize all body states to a JSON string.
+     *
+     * Format: {"t":<sim_time>,"vessels":[{"id":<n>,"x":...,"y":...,"z":...,"phi":...,"theta":...,"psi":...,"u":...,"v":...,"r":...},...]}
+     * Used by the WASM binding to transfer state to JavaScript.
+     */
+    std::string serializeStateJson() const;
+
     NavalDomain(const NavalDomain&)            = delete;
     NavalDomain& operator=(const NavalDomain&) = delete;
     NavalDomain(NavalDomain&&)                 = delete;

libs/simulation/src/NavalDomain.cpp

@@ -4,6 +4,7 @@
 
 #include <cassert>
 #include <cmath>
+#include <sstream>
 #include <stdexcept>
 
 namespace ymir {
@@ -188,4 +189,36 @@ naval::NavalContext NavalDomain::buildContext(int id, const BodyEntry& entry, do
                                env_->waterDepth(), env_->tide()};
 }
 
+std::string NavalDomain::serializeStateJson() const
+{
+    std::ostringstream os;
+    os << "{\"t\":" << sim_.time() << ",\"vessels\":[";
+
+    bool first = true;
+    for (const auto& [id, entry] : entries_)
+    {
+        const BodyState s = sim_.state(id);
+        const Vector6&  q = s.q();
+        const Vector6& qd = s.qdot();
+
+        if (!first) os << ',';
+        first = false;
+
+        os << "{\"id\":"   << id
+           << ",\"x\":"    << q[0]
+           << ",\"y\":"    << q[1]
+           << ",\"z\":"    << q[2]
+           << ",\"phi\":"  << q[3]
+           << ",\"theta\":" << q[4]
+           << ",\"psi\":"  << q[5]
+           << ",\"u\":"    << qd[0]
+           << ",\"v\":"    << qd[1]
+           << ",\"r\":"    << qd[5]
+           << '}';
+    }
+
+    os << "]}";
+    return os.str();
+}
+
 } // namespace ymir

tests/simulation/TestNavalDomain.cpp

@@ -282,3 +282,51 @@ TEST_CASE("NavalDomain regression: matches NavalSimulation within 1e-8 over 1000
         }
     }
 }
+
+// ---------------------------------------------------------------------------
+// serializeStateJson
+// ---------------------------------------------------------------------------
+
+TEST_CASE("NavalDomain: serializeStateJson produces valid JSON with expected keys", "[naval_domain]")
+{
+    ymir::Environment      env;
+    ymir::CouplingRegistry coupling;
+    ymir::NavalDomain      domain;
+
+    ymir::Vector6 q0{}, qd0{};
+    q0[0] = 5.0; q0[5] = 0.1;
+    domain.addBody(0, makeBody(0, 1e6, q0, qd0));
+    domain.onAddedToWorld(env, coupling);
+    domain.initialize();
+    domain.step(0.1);
+
+    const std::string json = domain.serializeStateJson();
+
+    REQUIRE(json.find("\"t\":") != std::string::npos);
+    REQUIRE(json.find("\"vessels\":") != std::string::npos);
+    REQUIRE(json.find("\"id\":0") != std::string::npos);
+    REQUIRE(json.find("\"x\":") != std::string::npos);
+    REQUIRE(json.find("\"psi\":") != std::string::npos);
+    REQUIRE(json.find("\"u\":") != std::string::npos);
+    REQUIRE(json.find("\"r\":") != std::string::npos);
+    REQUIRE(json.front() == '{');
+    REQUIRE(json.back()  == '}');
+}
+
+TEST_CASE("NavalDomain: serializeStateJson t advances with steps", "[naval_domain]")
+{
+    ymir::Environment      env;
+    ymir::CouplingRegistry coupling;
+    ymir::NavalDomain      domain;
+
+    domain.addBody(0, makeBody(0));
+    domain.onAddedToWorld(env, coupling);
+    domain.initialize();
+    domain.step(0.5);
+    domain.step(0.5);
+
+    const std::string json = domain.serializeStateJson();
+    const std::size_t tpos = json.find("\"t\":") + 4;
+    const double      t    = std::stod(json.substr(tpos));
+    REQUIRE(t == Approx(1.0).margin(1e-9));
+}