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.