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/common.h

@@ -0,0 +1,59 @@
+/* common.h — project-wide types, attributes, and error codes.
+ *
+ * Pure C11. No allocation, no platform calls; safe to include everywhere.
+ */
+#ifndef NAUT_COMMON_H
+#define NAUT_COMMON_H
+
+#include <stddef.h>
+#include <stdint.h>
+#include <stdbool.h>
+#include <stdatomic.h>
+
+/* --- sizes ------------------------------------------------------------- */
+#define NAUT_CACHELINE 64u
+#define NAUT_PAGE      4096u
+/* BitTorrent wire block size (BEP-3). The fundamental transfer unit. */
+#define NAUT_BLOCK     (16u * 1024u)
+
+/* --- compiler attributes ----------------------------------------------- */
+#define NAUT_LIKELY(x)      __builtin_expect(!!(x), 1)
+#define NAUT_UNLIKELY(x)    __builtin_expect(!!(x), 0)
+#define NAUT_INLINE         static inline __attribute__((always_inline))
+#define NAUT_ALIGNED(n)     __attribute__((aligned(n)))
+#define NAUT_CACHE_ALIGNED  __attribute__((aligned(NAUT_CACHELINE)))
+#define NAUT_NORETURN       __attribute__((noreturn))
+#define NAUT_UNUSED         __attribute__((unused))
+#define NAUT_PACKED         __attribute__((packed))
+#define NAUT_MUST_USE       __attribute__((warn_unused_result))
+#define NAUT_PRINTF(fi, ai) __attribute__((format(printf, fi, ai)))
+
+/* --- small helpers ----------------------------------------------------- */
+#define NAUT_ARRAY_LEN(a)   (sizeof(a) / sizeof((a)[0]))
+#define NAUT_MIN(a, b)      ((a) < (b) ? (a) : (b))
+#define NAUT_MAX(a, b)      ((a) > (b) ? (a) : (b))
+#define NAUT_ALIGN_UP(x, a)   (((uintptr_t)(x) + ((a) - 1)) & ~((uintptr_t)(a) - 1))
+#define NAUT_ALIGN_DOWN(x, a) ((uintptr_t)(x) & ~((uintptr_t)(a) - 1))
+#define NAUT_IS_POW2(x)     ((x) != 0 && (((x) & ((x) - 1)) == 0))
+#define NAUT_CONTAINER_OF(ptr, type, member) \
+    ((type *)((char *)(1 ? (ptr) : &((type *)0)->member) - offsetof(type, member)))
+
+/* --- error codes ------------------------------------------------------- */
+typedef int naut_err;
+enum {
+    NAUT_OK        = 0,
+    NAUT_ERR_NOMEM = -1,
+    NAUT_ERR_INVAL = -2,
+    NAUT_ERR_IO    = -3,
+    NAUT_ERR_AGAIN = -4,   /* would block / retry */
+    NAUT_ERR_PROTO = -5,   /* protocol violation  */
+    NAUT_ERR_RANGE = -6,
+    NAUT_ERR_NOSYS = -7,   /* unsupported by kernel/build */
+    NAUT_ERR_FULL  = -8,
+    NAUT_ERR_EMPTY = -9,
+    NAUT_ERR_NOTFOUND = -10,
+};
+
+const char *naut_strerror(naut_err e);
+
+#endif /* NAUT_COMMON_H */