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/log.h

@@ -0,0 +1,45 @@
+/* log.h — leveled logging.
+ *
+ * Phase 1: a straightforward thread-safe stderr logger (one writev per record,
+ * so lines never interleave). The data path does not log per-message; this is
+ * for lifecycle, errors, and stats. A lock-free per-thread ring drain is a
+ * later optimization behind the same macros, so call sites never change.
+ */
+#ifndef NAUT_LOG_H
+#define NAUT_LOG_H
+
+#include "naut/common.h"
+
+typedef enum {
+    NAUT_LOG_ERROR = 0,
+    NAUT_LOG_WARN,
+    NAUT_LOG_INFO,
+    NAUT_LOG_DEBUG,
+    NAUT_LOG_TRACE,
+} naut_log_level;
+
+void naut_log_set_level(naut_log_level lvl);
+naut_log_level naut_log_get_level(void);
+
+void naut_log_emit(naut_log_level lvl, const char *file, int line,
+                   const char *fmt, ...) NAUT_PRINTF(4, 5);
+
+#define NAUT_LOG(lvl, ...) \
+    do { if ((lvl) <= naut_log_get_level()) \
+        naut_log_emit((lvl), __FILE__, __LINE__, __VA_ARGS__); } while (0)
+
+#define NAUT_ERROR(...) NAUT_LOG(NAUT_LOG_ERROR, __VA_ARGS__)
+#define NAUT_WARN(...)  NAUT_LOG(NAUT_LOG_WARN,  __VA_ARGS__)
+#define NAUT_INFO(...)  NAUT_LOG(NAUT_LOG_INFO,  __VA_ARGS__)
+#define NAUT_DEBUG(...) NAUT_LOG(NAUT_LOG_DEBUG, __VA_ARGS__)
+#define NAUT_TRACE(...) NAUT_LOG(NAUT_LOG_TRACE, __VA_ARGS__)
+
+/* Fatal: log and abort(). Use only for unrecoverable invariant violations. */
+NAUT_NORETURN void naut_panic(const char *file, int line, const char *fmt, ...)
+    NAUT_PRINTF(3, 4);
+#define NAUT_PANIC(...) naut_panic(__FILE__, __LINE__, __VA_ARGS__)
+
+#define NAUT_ASSERT(cond) \
+    do { if (NAUT_UNLIKELY(!(cond))) NAUT_PANIC("assertion failed: %s", #cond); } while (0)
+
+#endif /* NAUT_LOG_H */