fix(skill_tree): UNC fallback para encontrar registry desde Windows nativo

Problema: skill_tree.exe en Windows tenia 0 nodos porque discover_registry_root
solo probaba env FN_REGISTRY_ROOT y walk-up desde cwd. Cwd en deploy es
C:\Users\lucas\Desktop\apps\skill_tree\ — sin registry.db cerca.

Fix:
- Anadir 3a estrategia (#ifdef _WIN32): probar paths UNC a WSL.
- Distros: WSL_REGISTRY_PATH env > Ubuntu-22.04 > Ubuntu > kali-linux > Debian.
- Tanto \wsl.localhost\<distro>\... como \wsl$\<distro>\... (legacy).
- Banner rojo en panel Tree si root vacio, con instrucciones.

Diagnostico:
- Mover discover + reload a first render frame (fn_log no inicializado
  antes de fn::run_app). Ahora log captura el path encontrado.

Verificacion: log dice
  discover_registry_root -> '\\wsl.localhost\Ubuntu-22.04\home\lucas\fn_registry'
  reloaded 173 nodes, 0 parse errors

Roadmap proximo: 0109m (issues_api HTTP service) reemplazara UNC con
cliente HTTP cuando este disponible.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

main.cpp

@@ -127,13 +127,36 @@ static const char* ring_label(int ring) {
 // ---- Registry root discovery -------------------------------------------
 
 static fs::path discover_registry_root() {
+    // 1. env var
     if (const char* env = std::getenv("FN_REGISTRY_ROOT")) {
         fs::path p(env);
         if (fs::exists(p / "registry.db")) return p;
     }
+    // 2. walk up desde cwd
     for (auto p = fs::current_path(); !p.empty() && p != p.root_path(); p = p.parent_path()) {
         if (fs::exists(p / "registry.db") && fs::exists(p / "dev" / "issues")) return p;
     }
+#ifdef _WIN32
+    // 3. fallback Windows: acceso UNC al sistema de archivos WSL. La ruta del
+    // distro/usuario se puede sobreescribir con WSL_REGISTRY_PATH para otros
+    // setups; defaults asumen Ubuntu + lucas (este PC).
+    const char* unc_env = std::getenv("WSL_REGISTRY_PATH");
+    std::vector<std::string> candidates;
+    if (unc_env && *unc_env) candidates.push_back(unc_env);
+    // Default WSL distros to probe (orden por probabilidad en este setup).
+    const char* distros[] = { "Ubuntu-22.04", "Ubuntu", "kali-linux", "Debian" };
+    for (const char* d : distros) {
+        candidates.push_back(std::string("\\\\wsl.localhost\\") + d + "\\home\\lucas\\fn_registry");
+        candidates.push_back(std::string("\\\\wsl$\\") + d + "\\home\\lucas\\fn_registry");
+    }
+    for (const auto& c : candidates) {
+        fs::path p(c);
+        std::error_code ec;
+        if (fs::exists(p / "registry.db", ec) && fs::exists(p / "dev" / "issues", ec)) {
+            return p;
+        }
+    }
+#endif
     return {};
 }
 
@@ -1022,6 +1045,22 @@ static void draw_tree() {
         return;
     }
 
+    // Banner si no encontramos registry root.
+    if (g_root.empty()) {
+        ImGui::PushStyleColor(ImGuiCol_ChildBg, IM_COL32(80, 20, 20, 255));
+        ImGui::BeginChild("root_err", ImVec2(0, 80), ImGuiChildFlags_Borders);
+        ImGui::TextColored(ImVec4(1, 0.6f, 0.6f, 1),
+            TI_ALERT_TRIANGLE " No se encontro registry root.");
+        ImGui::TextWrapped("Probadas: env FN_REGISTRY_ROOT, walk-up desde cwd"
+#ifdef _WIN32
+            ", env WSL_REGISTRY_PATH, \\\\wsl.localhost\\Ubuntu\\home\\lucas\\fn_registry, \\\\wsl$\\..."
+#endif
+            ". Setea FN_REGISTRY_ROOT (o WSL_REGISTRY_PATH en Win) y relanza."
+        );
+        ImGui::EndChild();
+        ImGui::PopStyleColor();
+    }
+
     // HUD strip.
     int n_done    = g_scan.count_by_status["completado"];
     int n_inprog  = g_scan.count_by_status["in-progress"];
@@ -1339,6 +1378,15 @@ static void draw_dashboard() {
 }
 
 static void render() {
+    // Defer first reload to first frame so fn_log is initialized.
+    static bool first_init = true;
+    if (first_init) {
+        first_init = false;
+        g_root = discover_registry_root();
+        fn_log::log_info("skill_tree: discover_registry_root -> '%s'",
+                         g_root.empty() ? "(empty)" : g_root.string().c_str());
+        reload_scan();
+    }
     if (g_show_tree)      draw_tree();
     if (g_show_inspector) draw_inspector();
     if (g_show_dashboard) draw_dashboard();
@@ -1388,8 +1436,8 @@ int main(int argc, char** argv) {
         if (std::strcmp(argv[i], "--self-test") == 0) return run_self_test();
     }
 
-    g_root = discover_registry_root();
-    reload_scan();
+    // discover_registry_root + reload_scan move to first render() frame
+    // (fn_log not initialized here yet — would lose discovery diagnostics).
 
     static fn_ui::PanelToggle panels[] = {
         { "Tree",      nullptr, &g_show_tree      },