Cleanup: drop boost::filesystem/thread/chrono, retire dead code

Migration from boost to std-library equivalents and removal of unreachable
code paths. Touches infrastructure only — no consensus rule or wallet
serialization changes.

Dead code removed:
- IRC bootstrap (irc.cpp/h, 417 lines): orphan from pre-Tor era, no callers.
- Alert system (alert.cpp/h + sendalert RPC + Qt UI signal, ~500 lines):
  retired post-V5 fork; old peers' alert messages now hit the unknown-cmd
  default branch, logged + ignored.
- Legacy P2P handlers in main.cpp: "checkpoint" (already a no-op stub since
  V5 fork master-key removal), "checkorder"/"reply" (2010-era Receive-by-IP
  feature), plus their unused supporting structures (CRequestTracker,
  PushRequest overloads, mapRequests/cs_mapRequests, mapReuseKey).
- Unreachable RPCs clearwallettransactions and scanforalltxns (~175 lines):
  defined in rpcwallet.cpp but never registered in the dispatch table.
- Stale -alertnotify CLI help text (option was advertised but never wired).

boost::filesystem -> std::filesystem (C++17):
- 30 source files, 5 headers. namespace fs = boost::filesystem swapped to
  namespace fs = std::filesystem; boost::filesystem::ifstream/ofstream
  replaced with std::ifstream/ofstream (path-aware in C++17);
  fs::system_complete -> fs::absolute; boost::filesystem::filesystem_error
  -> std::filesystem::filesystem_error.
- Build system: dropped Boost::filesystem from link libs and Boost
  components; PCH includes updated.
- Added explicit <filesystem> includes where types were previously
  available only transitively (db.h, rpcblockchain.cpp).

boost::thread -> std::thread (12 files):
- sync.h CCriticalSection/CWaitableCriticalSection now alias
  std::recursive_mutex/std::mutex. boost::unique_lock and
  boost::condition_variable / boost::mutex::scoped_lock swapped to std
  equivalents; sync.cpp boost::thread_specific_ptr -> thread_local
  std::unique_ptr.
- init.cpp boost::thread_group rewritten as std::vector<std::thread> with
  manual join loop. boost::thread::hardware_concurrency ->
  std::thread::hardware_concurrency.
- main.cpp/wallet.cpp -blocknotify/-walletnotify shell-out threads now use
  std::thread(...).detach() — fixes a latent bug where modern boost::thread
  destructor would call std::terminate on the joinable thread.
- util.cpp NewThread now catches std::system_error.
- No interruption_point/interrupt usage anywhere — pure mechanical swap.

boost::chrono / boost::posix_time -> std::chrono (3 of 5 files):
- util.h: MilliSleep, GetTimeMillis, GetTimeMicros rewritten on std::chrono
  (system_clock for epoch math, sleep_for for delays).
- snapshotnet.cpp: sleep_for swapped.
- DoS_tests.cpp: timing harness uses steady_clock.
- Skipped: rpcdump.cpp (boost::posix_time::time_input_facet has no clean
  std::get_time equivalent) and qt/qtipcserver.cpp (locked to
  boost::posix_time by boost::interprocess::message_queue::timed_receive).

Other housekeeping:
- Dropped unnecessary "using namespace boost;" from txdb-leveldb.cpp,
  txdb-rocksdb.cpp, walletdb.cpp, db.cpp (verified no unqualified boost
  names in those TUs).
- Removed unused extern declaration for clearwallettransactions.

Build fixes for non-unity builds on MinGW64/GCC 15:
- net.cpp: dropped stale #include "irc.h".
- addrman.cpp + main.cpp: explicit <cmath> include for sqrt/pow (was
  arriving transitively via boost headers).
- rpcblockchain.cpp + init.cpp: defensive #undef STRICT/ADVISORY/PERMISSIVE
  since windows.h macros collide with the Checkpoints:: enum values when
  std headers reorder include flow.
- tor_embed_hooks.cpp: triangles_tor_check_interrupted now polls fShutdown
  instead of boost::this_thread::interruption_requested (we never used
  boost interruption — the hook was always effectively a no-op).
- snapshotnet.cpp: fs::remove error handle uses std::error_code.
- serialize.h: added <ios> for std::ios::badbit/failbit (was relying on
  transitive include via boost).

Note: unity builds currently fail on this branch due to std::byte (C++17)
colliding with COM 'byte' typedef from shlobj.h when 'using namespace std;'
from earlier files in the unity slice leaks into util.cpp's parse of
shlobj.h. Build with -DENABLE_UNITY_BUILD=OFF (the default).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/bootstrap.cpp

@@ -4,8 +4,8 @@
 #include "bootstrap.h"
 #include "utxosnapshot.h"
 
-#include <boost/filesystem.hpp>
-#include <boost/filesystem/fstream.hpp>
+#include <filesystem>
+#include <fstream>
 #include <boost/algorithm/string.hpp>
 
 #include <zlib.h>
@@ -37,7 +37,7 @@
 extern bool fTestNet;
 namespace Checkpoints { bool IsKnownCheckpoint(int nHeight, const uint256& hash); }
 
-namespace fs = boost::filesystem;
+namespace fs = std::filesystem;
 
 namespace Bootstrap {