Initial commit: Naut-Torrent — from-scratch 10 GbE BitTorrent client

A maintainable, extensible BitTorrent client (C11, Linux/io_uring) targeting
10 GbE saturation. All torrent functionality is built from scratch; liburing
is the only linked third-party dependency on the data path.

Implements Phases 1-7 of the roadmap:
- core: page-aligned buffer pool, MPMC/Treiber queues, bitfields, worker pool
- crypto: SHA-1/256 (SHA-NI + scalar), Merkle (BEP-52), RC4 (MSE)
- bencode/metainfo: zero-copy parser, v1/v2/hybrid .torrent + magnet
- peer: sans-IO wire codec, MSE/PE handshake state machine, BEP-10, ut_metadata, PEX
- piece/storage: block-level multi-peer engine, rarest-first + endgame,
  per-file completion events + single-file relocate (move-as-you-finish)
- tracker/dht: HTTP + UDP (BEP-15) trackers, BEP-5 KRPC iterative lookup
- platform: io_uring reactor (SQPOLL, registered buffers, SEND_ZC)
- surface: versioned RPC, native plugin ABI, sandboxed Lua scripting, nautd/nautctl

Verified against libtorrent (single/multi/hybrid, MSE, magnet-via-DHT, swarm);
unit + interop tests green; ASan/UBSan/TSan clean. Scripting reference in
docs/scripting.md.

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

include/naut/session.h

@@ -0,0 +1,41 @@
+/* session.h — minimal torrent registry for the control plane.
+ *
+ * The control plane (RPC / plugins / scripts) refers to torrents by a stable
+ * uint64 id; the engine refers to them by their storage. This registry is the
+ * single map between the two, so a script-driven command like move_file can be
+ * resolved to a concrete naut_storage and acted on.
+ *
+ * Threading: the registry is *owner-thread confined*. In nautd every call
+ * (add/remove/move, all from RPC handlers and the move-drain) runs on the
+ * daemon's main thread, so no internal locking is needed. Commands originating
+ * on other threads (e.g. a script's naut.move_file) must be marshalled onto the
+ * owner thread first — which is exactly what nautd's bounded move queue does.
+ */
+#ifndef NAUT_SESSION_H
+#define NAUT_SESSION_H
+
+#include "naut/common.h"
+#include "naut/storage.h"
+
+typedef struct naut_session naut_session;
+
+naut_session *naut_session_create(void);
+/* Closes every storage still registered. */
+void          naut_session_destroy(naut_session *s);
+
+/* Register `storage` under `id`; the session takes ownership and will close it
+ * on remove/destroy. Fails with NAUT_ERR_INVAL if `id` is already present. */
+naut_err naut_session_add(naut_session *s, uint64_t id, naut_storage *storage);
+
+/* Close and forget the torrent. NAUT_ERR_NOTFOUND if unknown. */
+naut_err naut_session_remove(naut_session *s, uint64_t id);
+
+bool   naut_session_has(const naut_session *s, uint64_t id);
+size_t naut_session_count(const naut_session *s);
+
+/* Resolve `id` and relocate one completed file to `dest` (the storage half of
+ * "move files as they finish"). NAUT_ERR_NOTFOUND if the torrent is unknown. */
+naut_err naut_session_move_file(naut_session *s, uint64_t id,
+                                uint32_t file_index, const char *dest);
+
+#endif /* NAUT_SESSION_H */