test(daemon): add F1/F2 process-lifecycle integration tests; fix clobbered start error

Links the real EmbeddedDaemon into the ObsidianDragonTests target (its deps were
already present) and adds two POSIX integration tests that exercise the actual
fork/exec/waitpid fixes headlessly:

- testExecFailureReported (F2): start() against a non-executable file must fail with a
  precise "not executable or wrong architecture" reason.
- testDaemonCrashDetected (F1): a short-lived child that exits abnormally is still
  detected (crash_count_ increments) while isRunning() is hammered from the test
  thread — a regression test for the reap race.

Writing the F2 test surfaced a real bug: start()'s failure branch called
setState(State::Error, "Failed to start dragonxd process"), and setState stores the
Error message into last_error_ — clobbering the precise message startProcess() had
just set, so getLastError()/the UI only ever saw the generic string. Fixed to pass the
preserved detail to setState, so the precise reason survives and now also reaches the
state callback (crash panel / status).

ctest 1/1, green including the two new integration tests.

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

src/daemon/embedded_daemon.cpp

@@ -585,8 +585,14 @@ bool EmbeddedDaemon::start(const std::string& binary_path)
     override_extra_args_.clear();
 
     if (!startProcess(daemon_path, args)) {
-        DEBUG_LOGF("[ERROR] Failed to start dragonxd process: %s\\n", last_error_.c_str());
-        setState(State::Error, "Failed to start dragonxd process");
+        // startProcess() sets a precise last_error_ (e.g. "dragonxd could not be executed:
+        // ... not executable or wrong architecture"). Surface THAT via setState — which also
+        // stores the Error message into last_error_ — instead of clobbering it with a generic
+        // string that would then be all getLastError()/the UI ever sees.
+        std::string detail = last_error_.empty() ? std::string("Failed to start dragonxd process")
+                                                  : last_error_;
+        DEBUG_LOGF("[ERROR] %s\n", detail.c_str());
+        setState(State::Error, detail);
         return false;
     }