Cast file loses header/tail (0-byte or truncated .cast) when the recording process dies abnormally - StreamWriter buffering defeats incremental writes #6

Summary

The cast file is written through a buffered StreamWriter without AutoFlush and without a flush after the header line. If the recording host process dies abnormally (window closed, crash, taskkill), the recording loses everything still in the buffer - for short recordings that means a 0-byte cast file without even the JSON header, and for long recordings a file that ends mid-line with unterminated JSON. This contradicts the documented behavior that the cast "is written incrementally" and stays playable.

Environment

• Bsn.Host 0.1.0 (PowerShell Gallery)
• Windows 11 10.0.26300, PowerShell 7.6.9 host
• Killing the recording pwsh externally (or closing the console window) during a recording

Repro A - short recording loses everything

Start-InteractiveTranscript -Command 'ping -n 60 127.0.0.1' -Path .\x.cast
# from another window, a few seconds in:
Stop-Process -Name pwsh   # or close the console window

Result: x.cast is 0 bytes - the header (written by the CastWriter constructor) never reached disk because the default 1 KB character buffer was never filled and Dispose() never ran.

Repro B - long recording loses its tail

A ~75 s opencode TUI recording produced a 2.6 MB cast with 7384 events; the host was killed while the TUI was still running. The file's last line is truncated mid-JSON (Unterminated string passed in when parsing), so the final events are lost and strict parsers reject the file.

Root cause

src/Bsn.Host/src/PtyRecorder.cs, CastWriter:

writer = new StreamWriter(path, false, new UTF8Encoding(false));
writer.NewLine = "\n";
// header written, but never flushed

StreamWriter buffers ~1 KB of characters; nothing is flushed until the buffer fills or Dispose() runs. The engine's finally block only runs on orderly termination.

Suggested fix

• writer.Flush() (or writer.AutoFlush = true) immediately after writing the header line, so a valid header is always on disk.
• AutoFlush = true for event lines (they are small and latency-tolerant), or at minimum flush after each event. The volume is trivial compared to correctness for an audit-oriented feature.

With per-line flushing, an abnormally terminated recording stays a valid asciicast file that simply ends at the last completed event.