Fix GUI freeze on stop and prevent corrupted XDF footers

[sappelhoff]

Problem

When stopping a recording, recording::~recording() executes synchronously on the Qt UI thread. Because record_offsets() used an uninterruptible 5-second sleep (std::this_thread::sleep_for(offset_interval)) and boundary_thread joined with a 15-second timeout, the Qt event loop blocked, causing Windows to mark the window as "(Not Responding)" for 5–15+ seconds. Forcefully terminating the application when frozen aborted before stream footers could be written, resulting in corrupted XDF footers and truncated files.

Solution

1. Interruptible Thread Teardown: Added a std::condition_variable shutdown_cv_ in recording and replaced std::this_thread::sleep_for / sleep_until across record_offsets(), record_boundaries(), and typed_transfer_loop() with condition variable waits. When shutdown is triggered, all worker threads wake up in < 1 ms.
2. Active Socket Teardown: Track active lsl::stream_inlet instances and invoke in->close_stream() on shutdown to abort any blocking TCP socket calls or unreachable remote network routes immediately.
3. Low-Latency First-Sample Check: Lowered the initial in->pull_sample() timeout from 4.0s to 0.1s so silent or late-starting streams check shutdown_ rapidly.
4. Automated Integration Test: Added scripts/test_recording_teardown.py to verify that LabRecorderCLI stops in < 0.5s and produces fully valid, uncorrupted XDF files with intact footers.

Verification

• Tested on Windows and Ubuntu via GitHub Actions CI and local automated integration tests.
• Confirmed teardown completes in ~0.5s and resulting .xdf parses cleanly with pyxdf.

[cboulay]

Thanks for tackling the stop latency. I recommend addressing these before merging:

1. Missed shutdown notifications (high priority). shutdown_ and offset_shutdown are changed without holding shutdown_mut_, while the condition-variable predicates use that mutex. A notifier can set the flag and call notify_all() after the waiter reads false but before it enters the wait. The offset thread can then sleep for the full five-second interval. I reproduced this scheduling pattern in a standalone C++ harness. Update the predicate state under the same mutex, then notify, including both the normal and exception paths.

2. The unreachable-stream stop hang is not fully resolved. close_stream() in liblsl v1.18.0.b3 stops the data receiver; it does not cancel the separate metadata receiver. record_from_streaminfo() still calls in->info() with its default infinite timeout, including after an interrupted open_stream(). With an unreachable metadata endpoint, Stop can therefore still block indefinitely. Also, the existing try_join_once() calls blocking std::thread::join(), so the reduced max_join_wait does not bound this wait. Please use bounded metadata waits that observe shutdown and explicitly handle stopping during subscription/header retrieval.

3. Validation does not cover these guarantees yet. The new integration script is not called by the checked-in CI workflow. It verifies only the EEG footer, not the silent marker stream's footer, and permits 1.5 seconds despite documenting a 500 ms limit. Please run it in CI, check both footers and process exit status, and add stop-during-connect/metadata and repeated shutdown cases.

Review scope: source inspection against the PR head and liblsl v1.18.0.b3, plus the isolated condition-variable reproduction; I did not run the full integration script.