feat(diagnostics): make the logging + crash infrastructure actually work (W7-2, W7-3, W7-4)

The Foundation tier — answers the original "make it easier to diagnose problems" ask.

- W7-2 (keystone): the app-level Logger file sink was never initialized, so LOG/LOGF/
  VERBOSE_LOGF went nowhere and dragonx-debug.log didn't exist on Linux/macOS at all.
  main() now calls Logger::init(<config>/dragonx-debug.log) on every platform. Fixed a
  latent deadlock this exposed: init() wrote its banner via write(), which re-locks the
  non-recursive mutex_ it already holds — now written directly. On Windows the raw
  stdout/stderr freopen moved to a separate dragonx-stdout.log so the two writers don't
  contend on one file. Added testLoggerFileSink (also a deadlock guard — it would hang if
  the fix regressed).

- W7-3: no crash handler existed on Linux/macOS. Added an async-signal-safe sigaction
  handler (SIGSEGV/ABRT/BUS/FPE/ILL) that writes the signal id + a backtrace_symbols_fd
  backtrace to dragonx-crash.log, then re-raises the default disposition for a core dump —
  the POSIX counterpart of the Windows SEH filter.

- W7-4: Logger::init now rotates the log to a single .1 backup past 10 MB, so a long or
  verbose session can't grow it unbounded.

Build-clean; ctest 1/1. Remaining Foundation: the QoL bundle (mostly UI). See
docs/wallet-hardening.md.

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

tests/test_phase4.cpp

@@ -5,6 +5,7 @@
 #include "daemon/daemon_controller.h"
 #include "daemon/embedded_daemon.h"
 #include "util/connect_stall.h"
+#include "util/logger.h"
 #include "data/transaction_history_cache.h"
 #include "data/address_book.h"
 #include "data/wallet_index.h"
@@ -2546,6 +2547,33 @@ void testConsoleSecretRedaction()
     EXPECT_EQ(RedactConsoleCommand("getwalletinfo"),              std::string("getwalletinfo"));
 }
 
+void testLoggerFileSink()
+{
+    using dragonx::util::Logger;
+    namespace fsn = std::filesystem;
+    fsn::path logPath = fsn::temp_directory_path() / "od_logger_test.log";
+    std::error_code ec;
+    fsn::remove(logPath, ec);
+    fsn::remove(logPath.string() + ".1", ec);
+
+    // W7-2: init() opens the file sink and must NOT deadlock — it writes the banner under the same
+    // non-recursive lock it holds (this test would hang if that regressed).
+    Logger& lg = Logger::instance();
+    EXPECT_TRUE(lg.init(logPath.string()));
+    lg.write("hello-w7-2-sink");
+    EXPECT_TRUE(fsn::exists(logPath));
+
+    std::ifstream f(logPath.string());
+    std::string all, line;
+    while (std::getline(f, line)) all += line + "\n";
+    f.close();
+    EXPECT_TRUE(all.find("hello-w7-2-sink") != std::string::npos);
+    EXPECT_TRUE(all.find("Logger initialized") != std::string::npos);
+
+    fsn::remove(logPath, ec);
+    fsn::remove(logPath.string() + ".1", ec);
+}
+
 void testConnectHasStalled()
 {
     using dragonx::util::connectHasStalled;
@@ -6906,6 +6934,7 @@ int main()
     testIsLocalHost();
     testAllowsPlaintextRemote();
     testConsoleSecretRedaction();
+    testLoggerFileSink();
     testDaemonLifecycleExecution();
     testDaemonLifecycleAdapters();
     testConsoleTextLayout();