Initial commit: Build project architecture, define interfaces, write documentation, with the help of Claude

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+/*
+ * richard-atp-engine: A saturation-based automated theorem prover for first-order logic.
+ * Copyright (C) 2026 Richard Qin
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+
+#pragma once
+
+/// @file term.h
+/// @brief Core Term data structure — hash-consed, immutable.
+
+#include <span>
+
+#include "atp/core/types.h"
+
+namespace atp {
+
+/// A term is a function/predicate application: f(t1, t2, ..., tn).
+/// Stored in a flat arena; args are contiguous TermIds following the header.
+/// Variables are represented as terms with a special symbol range.
+struct Term {
+    SymbolId symbol_id;  ///< Index into the SymbolTable
+    uint16_t arity;      ///< Number of arguments
+
+    /// Access the argument TermIds (stored immediately after this struct in the arena).
+    [[nodiscard]] std::span<const TermId> args() const;
+    [[nodiscard]] std::span<TermId> mutableArgs();
+};
+
+}  // namespace atp