Tell a second person who has a project open, and open it read-only

Two people opening the same project on a shared drive could both edit it,
and the last save silently wiped the other's work. A project now has a
hidden lock file beside it (.~plant.qetz.lock, small JSON: user, host,
process, version, since, heartbeat). Opening a project another live session
holds shows who has it, since when and how long ago they were last active,
and offers Open read-only (default), Open anyway, or Cancel.

- The lock is made with an exclusive create, refreshed every minute and
  deleted when the project closes. It is not a QLockFile: that treats any
  lock from another machine as stale after 30 s. Here only a lock made on
  this machine by a process that no longer exists is taken silently.
- A folder the lock cannot be made in, or the setting turned off
  (Settings > General > Projects), opens as before.
- Save As and .qet saved as .qetz move the lock to the new file, and refuse
  a file someone else has open. A project opened read-only for another's
  lock does not save over that file.
- --resave and --set-titleblock refuse a file someone has open unless given
  --ignore-lock; the read-only verbs never look at locks and headless runs
  never take one. A script's qet.save() refuses the same way.

Checked: 17 new unit tests (5 go red when stale detection and the exclusive
create are broken), full ctest 107/107, the dialog and both choices in the
GUI under Xvfb, and --resave refused/allowed from the command line.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/main.cpp

@@ -240,6 +240,9 @@ int main(int argc, char **argv)
 			// runs on a background thread referencing the project and races the
 			// process exit (intermittent segfault in QET::writeToFile).
 			QETProject::setBackupEnabled(false);
+			// Headless runs read locks, to refuse writing a project someone
+			// has open, but never take one
+			ProjectLock::setHeadless(true);
 			// Answer message boxes instead of showing them: opening a project
 			// saved by an older QElectroTech raises a warning from
 			// QETProject::readProjectXml(), and with nobody able to dismiss it
@@ -255,6 +258,7 @@ int main(int argc, char **argv)
 			installHeadlessMessageHandler();
 			QApplication script_app(argc, argv);
 			QETProject::setBackupEnabled(false);
+			ProjectLock::setHeadless(true);
 			QET::QetMessageBox::setNonInteractive(true);
 			return QetScripting::run(headlessArguments(script_app.arguments()));
 		}