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/naut_plugin.h

@@ -0,0 +1,49 @@
+/* naut_plugin.h - stable versioned native plugin ABI. */
+#ifndef NAUT_PLUGIN_H
+#define NAUT_PLUGIN_H
+
+#include "naut/common.h"
+#include "naut/event.h"
+
+#define NAUT_PLUGIN_ABI_VERSION 1u
+
+typedef struct {
+    uint32_t abi_version;
+    uint32_t struct_size;
+    const char *name;
+    void *(*open)(const char *root, naut_err *error);
+    void (*close)(void *storage);
+    naut_err (*read)(void *storage, int64_t offset, void *buffer, size_t length);
+    naut_err (*write)(void *storage, int64_t offset,
+                      const void *buffer, size_t length);
+} naut_storage_backend_v1;
+
+/* request_json is a JSON object or null. response_json must be malloc-owned
+ * compact JSON on success; the host frees it after parsing. */
+typedef naut_err (*naut_plugin_rpc_fn)(void *context,
+                                       const char *request_json,
+                                       char **response_json);
+typedef void (*naut_plugin_event_fn)(void *context,
+                                     const naut_event *event);
+
+typedef struct naut_host_api {
+    uint32_t abi_version;
+    uint32_t struct_size;
+    void *host_context;
+
+    naut_err (*set_plugin_name)(void *host_context, const char *name);
+    naut_err (*register_rpc)(void *host_context, const char *method,
+                             naut_plugin_rpc_fn callback, void *context);
+    naut_err (*register_storage_backend)(
+        void *host_context, const naut_storage_backend_v1 *backend);
+    naut_err (*subscribe_event)(void *host_context,
+                                naut_plugin_event_fn callback,
+                                void *context);
+    void (*emit_event)(void *host_context, const naut_event *event);
+    void (*log)(void *host_context, int level, const char *message);
+} naut_host_api;
+
+/* Every plugin exports this exact symbol. */
+typedef naut_err (*naut_plugin_register_fn)(const naut_host_api *host);
+
+#endif /* NAUT_PLUGIN_H */