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/mpmc.h

@@ -0,0 +1,34 @@
+/* mpmc.h — bounded lock-free queue of pointers (Dmitry Vyukov's algorithm).
+ *
+ * One implementation covers every cross-thread hand-off in the engine:
+ *   - control-plane -> reactor command queues (MPSC usage)
+ *   - reactor -> hash worker pool job queue   (MPMC usage)
+ *   - worker -> reactor completion queue       (MPSC usage)
+ * Wait-free in the common case, no per-op allocation. Capacity is fixed at
+ * init and must be a power of two.
+ */
+#ifndef NAUT_MPMC_H
+#define NAUT_MPMC_H
+
+#include "naut/common.h"
+
+typedef struct naut_mpmc_cell {
+    _Atomic size_t seq;
+    void          *data;
+} naut_mpmc_cell;
+
+typedef struct naut_mpmc {
+    NAUT_CACHE_ALIGNED naut_mpmc_cell *buffer;
+    size_t mask;
+    NAUT_CACHE_ALIGNED _Atomic size_t enqueue_pos;
+    NAUT_CACHE_ALIGNED _Atomic size_t dequeue_pos;
+} naut_mpmc;
+
+naut_err naut_mpmc_init(naut_mpmc *q, size_t capacity_pow2);
+void     naut_mpmc_destroy(naut_mpmc *q);
+
+/* Both return false without blocking when full/empty respectively. */
+bool naut_mpmc_push(naut_mpmc *q, void *p);
+bool naut_mpmc_pop(naut_mpmc *q, void **out);
+
+#endif /* NAUT_MPMC_H */