feat(solver-bootstrap): step 3 — core-ids-and-diagnostics

include/fesa/core/entity_id.hpp

@@ -0,0 +1,28 @@
+#pragma once
+
+#include <compare>
+#include <cstdint>
+#include <stdexcept>
+
+namespace fesa {
+
+template <class Tag>
+class EntityId final {
+public:
+    explicit constexpr EntityId(const std::int64_t value) : value_{value} {
+        if (value < 0) {
+            throw std::invalid_argument{"EntityId value must not be negative."};
+        }
+    }
+
+    [[nodiscard]] constexpr std::int64_t value() const noexcept {
+        return value_;
+    }
+
+    auto operator<=>(const EntityId&) const = default;
+
+private:
+    std::int64_t value_;
+};
+
+}  // namespace fesa