Initial commit: multi-peer torrent download engine

Reactor/loop-pool engine with TCP/µTP/MSE transports, per-connection
pipelining, priority-driven piece selection with endgame, and the Python
FFI test harness.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CMakeLists.txt

@@ -0,0 +1,58 @@
+cmake_minimum_required(VERSION 3.16)
+project(torrent_peer C)
+
+set(CMAKE_C_STANDARD 11)
+set(CMAKE_C_STANDARD_REQUIRED ON)
+
+if(NOT CMAKE_BUILD_TYPE)
+    set(CMAKE_BUILD_TYPE Release)
+endif()
+
+# Fast by default; -march=native is great for local benchmarking. Disable with
+# -DPEER_NATIVE=OFF for portable / CI builds.
+option(PEER_NATIVE "Optimize for the build host (-march=native)" ON)
+option(PEER_ASAN "Build with AddressSanitizer/UBSan" OFF)
+
+add_library(torrentpeer SHARED
+    src/engine.c
+    src/loop.c
+    src/connection.c
+    src/transport.c
+    src/transport_mse.c
+    src/transport_utp.c
+    src/crypto.c
+    src/proto.c
+    src/scheduler.c
+    src/reqtab.c
+    src/ring.c
+    src/arena.c
+    src/peer_compat.c
+)
+target_include_directories(torrentpeer PUBLIC include)
+
+target_compile_options(torrentpeer PRIVATE
+    -O3 -Wall -Wextra -Wno-unused-parameter
+)
+if(PEER_NATIVE AND NOT PEER_ASAN)
+    target_compile_options(torrentpeer PRIVATE -march=native)
+endif()
+if(PEER_ASAN)
+    target_compile_options(torrentpeer PRIVATE -O1 -g -fsanitize=address,undefined -fno-omit-frame-pointer)
+    target_link_options(torrentpeer PRIVATE -fsanitize=address,undefined)
+endif()
+
+find_package(Threads REQUIRED)
+target_link_libraries(torrentpeer PRIVATE Threads::Threads)
+
+# P3a: link-time optimization for release builds (skipped under ASan).
+option(PEER_LTO "Enable link-time optimization" ON)
+if(PEER_LTO AND NOT PEER_ASAN)
+    include(CheckIPOSupported)
+    check_ipo_supported(RESULT _ipo_ok OUTPUT _ipo_msg)
+    if(_ipo_ok)
+        set_target_properties(torrentpeer PROPERTIES INTERPROCEDURAL_OPTIMIZATION ON)
+    endif()
+endif()
+
+# Keep the .so name predictable for ctypes: libtorrentpeer.so
+set_target_properties(torrentpeer PROPERTIES OUTPUT_NAME torrentpeer)