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/merkle.h

@@ -0,0 +1,40 @@
+/* merkle.h — BitTorrent v2 (BEP-52) SHA-256 Merkle trees.
+ *
+ * In v2 each file is split into 16 KiB leaf blocks; the leaf hashes form a
+ * binary Merkle tree whose interior nodes are SHA-256(left || right). When the
+ * leaf count is not a power of two, the tree is padded with *zero hashes* (a
+ * block of 32 zero bytes at the leaf level, then SHA-256 of two children up the
+ * tree) so the shape is a perfect binary tree. The root at the "piece layer"
+ * boundary gives per-piece verifiability; the whole-file root goes in the
+ * metainfo file tree.
+ *
+ * This module provides the tree primitive; metainfo/storage wire it to files.
+ */
+#ifndef NAUT_MERKLE_H
+#define NAUT_MERKLE_H
+
+#include "naut/common.h"
+#include "naut/hash.h"
+
+#define NAUT_MERKLE_LEAF (16u * 1024u)   /* BEP-52 block size */
+
+/* Compute the Merkle root of `nleaves` 32-byte leaf hashes, padding up to the
+ * next power of two with zero hashes. nleaves==0 yields the all-zero hash.
+ * `leaves` is nleaves*32 bytes; out is 32 bytes. Scratch is allocated
+ * internally. Returns NAUT_OK or NAUT_ERR_NOMEM. */
+naut_err naut_merkle_root(const uint8_t *leaves, size_t nleaves,
+                          uint8_t out[NAUT_SHA256_LEN]);
+
+/* As above but pad to a fixed `block_count` (>= nleaves, power of two) rather
+ * than the next power of two — used to compute a piece-layer root where the
+ * tree height is fixed by the piece size. */
+naut_err naut_merkle_root_padded(const uint8_t *leaves, size_t nleaves,
+                                 size_t block_count,
+                                 uint8_t out[NAUT_SHA256_LEN]);
+
+/* Hash a contiguous data buffer into leaf hashes (one SHA-256 per 16 KiB, last
+ * leaf may be short). out must hold ceil(len/16KiB)*32 bytes. Returns the leaf
+ * count. */
+size_t naut_merkle_leaves(const uint8_t *data, size_t len, uint8_t *out);
+
+#endif /* NAUT_MERKLE_H */