writer: detect overwrite conflict at start for back-channel transports

Previously an existing output file was only detected at the final rename,
after the whole collection had run. Add an up-front check in the FileWriter
constructor that fails before acquisition arms.

Only the master file is checked, and only by the single writer that owns it
(write_master_file). In a multi-writer TCP/ZMQ setup the per-image data files
are staggered across writers by file number, and a writer does not even know
the total writer count, so it cannot enumerate its own subset; checking all
data files in every writer would make each writer stat files it never writes
and race the writers already creating them. Data-file conflicts stay caught by
their owning writer at the final rename.

Gate the check on whether the transport can report the failure to the broker:
the direct HDF5 pusher throws in-process and the TCP writer returns a
START-failure ACK, so both abort the collection cleanly (verified: an asymmetric
rejection cancels the already-started sibling). The ZeroMQ path is fire-and-forget
with no back-channel, so StreamWriter opts out via image_puller.SupportsAck() and
keeps writing .tmp files, failing only at the final rename (leaving the images on
disk). Documented in docs/JFJOCH_WRITER.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

writer/StreamWriter.cpp

@@ -66,7 +66,11 @@ void StreamWriter::ProcessStartMessage() {
     writer_notification_zmq_addr = image_puller_output.cbor->start_message->writer_notification_zmq_addr;
 
     try {
-        file_writer = std::make_unique<FileWriter>(*image_puller_output.cbor->start_message);
+        // Fail fast on an overwrite conflict only when the transport can report it
+        // back to the broker (TCP ACK). The ZeroMQ path has no back-channel, so it
+        // keeps writing .tmp files and fails at the final rename instead.
+        file_writer = std::make_unique<FileWriter>(*image_puller_output.cbor->start_message,
+                                                   image_puller.SupportsAck());
         if (!file_done_address.empty())
             file_writer->SetupFinalizedFileSocket(file_done_address);
         logger.Info("Starting writing for dataset {} of {} images",