core/ui: extract export into src/app (frontend-agnostic), thin TUI command

First step of separating business logic from the TUI. The export command built
the CSV/ODS file inside its lambda, mixed with Print/ShowError/dialog calls.
Move all of it — CSV + ODS building, sheet-name sanitising, file writing — into
src/app/export.{hpp,cpp} (namespace app, no FTXUI/console dependency):
export_connections(const System*, path, format) -> ExportResult. The TUI
command is now a thin wrapper (resolve args/dialog, call the core, render). The
core is unit-tested without any UI (test_export); 342 assertions pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_export.cpp

@@ -0,0 +1,76 @@
+#include <doctest/doctest.h>
+
+#include "app/export.hpp"
+#include "system/connect.hpp"
+#include "system/modules.hpp"
+#include "system/parts.hpp"
+#include "system/pins.hpp"
+#include "system/signals.hpp"
+#include "system/system.hpp"
+
+#include <cstdio>
+#include <fstream>
+#include <sstream>
+#include <string>
+
+namespace {
+
+std::string slurp(const std::string &path)
+{
+    std::ifstream f(path);
+    std::stringstream ss;
+    ss << f.rdbuf();
+    return ss.str();
+}
+
+}  // namespace
+
+TEST_CASE("export_format_from_path maps extensions") {
+    app::ExportFormat f;
+    CHECK(app::export_format_from_path("a.csv", f));
+    CHECK(f == app::ExportFormat::Csv);
+    CHECK(app::export_format_from_path("a.ODS", f));
+    CHECK(f == app::ExportFormat::Ods);
+    CHECK_FALSE(app::export_format_from_path("a.txt", f));
+    CHECK_FALSE(app::export_format_from_path("noext", f));
+}
+
+TEST_CASE("export_connections writes a flat CSV (no UI needed)") {
+    // Two cards, one wired pin pair via a connection.
+    System sys;
+    Module *a = sys.modules()->merge("A");
+    Module *b = sys.modules()->merge("B");
+    Part *ja = new Part("J1"); a->add(ja);
+    Part *jb = new Part("P1"); b->add(jb);
+    Pin *pa = new Pin("1"); ja->add(pa);
+    Pin *pb = new Pin("1"); jb->add(pb);
+    Signal *sa = a->signals->merge("NETA"); sa->add(pa); pa->connect(sa);
+    Signal *sb = b->signals->merge("NETB"); sb->add(pb); pb->connect(sb);
+
+    Connection *c = new Connection("A.J1<->B.P1", a, ja, b, jb);
+    c->transform_name = "identity";
+    c->pin_map.emplace_back(pa, pb);
+    sys.connections()->add(c);
+
+    const char *path = "test_export_out.csv";
+    app::ExportResult r = app::export_connections(&sys, path, app::ExportFormat::Csv);
+    CHECK(r.ok);
+    CHECK(r.rows == 1);
+
+    std::string out = slurp(path);
+    CHECK(out.find("connection,transform,") == 0);           // header present
+    CHECK(out.find("A.J1<->B.P1") != std::string::npos);     // connection name
+    CHECK(out.find("identity") != std::string::npos);        // transform
+    CHECK(out.find("NETA") != std::string::npos);            // left signal
+    CHECK(out.find("NETB") != std::string::npos);            // right signal
+
+    std::remove(path);
+}
+
+TEST_CASE("export_connections reports a bad path instead of crashing") {
+    System sys;
+    app::ExportResult r = app::export_connections(
+        &sys, "/nonexistent-dir-xyz/out.csv", app::ExportFormat::Csv);
+    CHECK_FALSE(r.ok);
+    CHECK_FALSE(r.error.empty());
+}