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>

tests/unit/test_extension.c

@@ -0,0 +1,87 @@
+#include "naut/extension.h"
+#include "naut/peer.h"
+#include "test.h"
+
+#include <stdlib.h>
+#include <string.h>
+
+static const uint8_t *ext_payload(const uint8_t *frame, size_t len,
+                                  uint8_t *ext_id, size_t *payload_len) {
+    naut_msg msg;
+    CHECK(naut_peer_msg_parse(frame, len, &msg) == (int)len);
+    CHECK(msg.type == NAUT_MSG_EXTENDED && msg.payload_len >= 1);
+    *ext_id = msg.payload[0];
+    *payload_len = msg.payload_len - 1;
+    return msg.payload + 1;
+}
+
+int main(void) {
+    uint8_t *frame = NULL;
+    size_t frame_len = 0, payload_len = 0;
+    uint8_t ext_id = 0;
+
+    CHECK(naut_ext_build_handshake(NAUT_EXT_UT_METADATA, NAUT_EXT_UT_PEX,
+                                   31235, 6881, &frame, &frame_len) == NAUT_OK);
+    const uint8_t *payload = ext_payload(frame, frame_len, &ext_id, &payload_len);
+    CHECK_EQ(ext_id, 0);
+    naut_ext_handshake hs;
+    CHECK(naut_ext_parse_handshake(payload, payload_len, &hs) == NAUT_OK);
+    CHECK_EQ(hs.ut_metadata, NAUT_EXT_UT_METADATA);
+    CHECK_EQ(hs.ut_pex, NAUT_EXT_UT_PEX);
+    CHECK_EQ(hs.metadata_size, 31235);
+    CHECK_EQ(hs.port, 6881);
+    CHECK_EQ(hs.reqq, 256);
+    free(frame);
+
+    CHECK(naut_metadata_build(3, NAUT_METADATA_REQUEST, 7, 0, NULL, 0,
+                              &frame, &frame_len) == NAUT_OK);
+    payload = ext_payload(frame, frame_len, &ext_id, &payload_len);
+    CHECK_EQ(ext_id, 3);
+    naut_metadata_msg mm;
+    CHECK(naut_metadata_parse(payload, payload_len, &mm) == NAUT_OK);
+    CHECK(mm.type == NAUT_METADATA_REQUEST && mm.piece == 7 && mm.data_len == 0);
+    free(frame);
+
+    uint8_t metadata[100];
+    for (size_t i = 0; i < sizeof metadata; i++) metadata[i] = (uint8_t)i;
+    CHECK(naut_metadata_build(3, NAUT_METADATA_DATA, 1,
+                              NAUT_METADATA_BLOCK + sizeof metadata,
+                              metadata, sizeof metadata,
+                              &frame, &frame_len) == NAUT_OK);
+    payload = ext_payload(frame, frame_len, &ext_id, &payload_len);
+    CHECK(naut_metadata_parse(payload, payload_len, &mm) == NAUT_OK);
+    CHECK(mm.type == NAUT_METADATA_DATA && mm.piece == 1);
+    CHECK_EQ(mm.total_size, NAUT_METADATA_BLOCK + sizeof metadata);
+    CHECK_EQ(mm.data_len, sizeof metadata);
+    CHECK(memcmp(mm.data, metadata, sizeof metadata) == 0);
+    free(frame);
+
+    naut_peer_addr added[2] = {
+        { { 1, 2, 3, 4 }, 6881 },
+        { { 10, 0, 0, 2 }, 51413 },
+    };
+    naut_peer_addr dropped[1] = { { { 192, 0, 2, 7 }, 7000 } };
+    uint8_t flags[2] = { 0x01, 0x12 };
+    naut_pex_msg pex = {
+        .added = added, .added_flags = flags, .num_added = 2,
+        .dropped = dropped, .num_dropped = 1,
+    };
+    CHECK(naut_pex_build(4, &pex, &frame, &frame_len) == NAUT_OK);
+    payload = ext_payload(frame, frame_len, &ext_id, &payload_len);
+    CHECK_EQ(ext_id, 4);
+    naut_pex_msg parsed;
+    CHECK(naut_pex_parse(payload, payload_len, &parsed) == NAUT_OK);
+    CHECK_EQ(parsed.num_added, 2);
+    CHECK_EQ(parsed.num_dropped, 1);
+    CHECK(parsed.added[0].port == 6881 && parsed.added[1].port == 51413);
+    CHECK(parsed.added_flags[0] == 0x01 && parsed.added_flags[1] == 0x12);
+    CHECK(parsed.dropped[0].ip[0] == 192 && parsed.dropped[0].port == 7000);
+    naut_pex_free(&parsed);
+    free(frame);
+
+    const uint8_t malformed[] = "d5:added5:abcdee";
+    CHECK(naut_pex_parse(malformed, sizeof malformed - 1, &parsed) ==
+          NAUT_ERR_PROTO);
+
+    TEST_MAIN_END();
+}