[FEATURE] Allow output \0 terminated frames (for WebSocket streaming support)

[pszemus]

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When streaming subtitles (particularly DVBSUB) from ccextractor to WebSocket endpoints via tools like websocat, multi-line subtitles cause issues. Each line is sent as a separate message, resulting in only the last line being visible at the receiving end.

For example, using the following pipeline:

ccextractor --udp <src_stream_address> --codec dvbsub --out=txt --stdout --forceflush | websocat ws://<endpoint-uri>

multi-line subtitle frames are sent line-by-line, losing all but the final line.

This PR introduces the --null-terminated option, which appends a null character (\0) as a frame delimiter after each complete subtitle frame (whether single or multi-line). This enables proper frame boundaries for streaming scenarios.

Then, it'll be possible to create the following pipeline:

ccextractor --udp <src_stream_address> --codec dvbsub --out=txt --null-terminated --stdout --forceflush | websocat -0 ws://<endpoint-uri>

With this change, websocat's -0 flag can properly parse complete subtitle frames using the null delimiter (see websocat documentation).

Benefits:

• Enables reliable WebSocket streaming of subtitles without data loss
• Maintains backward compatibility (opt-in feature)
• Follows established patterns for null-terminated stream processing
• Simple, focused change that solves a real-world use case

Please compare the following two output files, where with --null-terminated enabled new lines in multi-line subtitles were preserved and all frames end with \0.

• --out=webvtt:
  ccextractor_webvtt.txt
• --out=txt --null-terminated:
  ccextractor_txt_null-terminated.txt

[ccextractor-bot]

CCExtractor CI platform finished running the test files on linux. Below is a summary of the test results, when compared to test for commit dd29311...:

Report Name	Tests Passed
Broken	13/13
CEA-708	14/14
DVB	6/7
DVD	3/3
DVR-MS	2/2
General	27/27
Hardsubx	1/1
Hauppage	3/3
MP4	3/3
NoCC	10/10
Options	85/86
Teletext	21/21
WTV	13/13
XDS	34/34

Your PR breaks these cases:

• ccextractor --autoprogram --out=srt --latin1 --quant 0 85271be4d2...
• ccextractor --out=spupng c83f765c66...

Congratulations: Merging this PR would fix the following tests:

• ccextractor --autoprogram --out=ttxt --latin1 --ucla dab1c1bd65..., Last passed: Never
• ccextractor --out=srt --latin1 --autoprogram 29e5ffd34b..., Last passed: Never
• ccextractor --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsnotbefore 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsnotafter 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsforatleast 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never
• ccextractor --startcreditsforatmost 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9..., Last passed: Never

It seems that not all tests were passed completely. This is an indication that the output of some files is not as expected (but might be according to you).

Check the result page for more info.

[ccextractor-bot]

CCExtractor CI platform finished running the test files on windows. Below is a summary of the test results, when compared to test for commit dd29311...:

Report Name	Tests Passed
Broken	13/13
CEA-708	14/14
DVB	6/7
DVD	3/3
DVR-MS	2/2
General	25/27
Hardsubx	1/1
Hauppage	3/3
MP4	3/3
NoCC	10/10
Options	81/86
Teletext	21/21
WTV	13/13
XDS	34/34

Your PR breaks these cases:

• ccextractor --autoprogram --out=srt --latin1 --quant 0 85271be4d2...
• ccextractor --autoprogram --out=ttxt --latin1 --ucla dab1c1bd65...
• ccextractor --out=srt --latin1 --autoprogram 29e5ffd34b...
• ccextractor --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...
• ccextractor --startcreditsnotbefore 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...
• ccextractor --startcreditsnotafter 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...
• ccextractor --startcreditsforatleast 1 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...
• ccextractor --startcreditsforatmost 2 --startcreditstext "CCextractor Start crdit Testing" c4dd893cb9...

Congratulations: Merging this PR would fix the following tests:

• ccextractor --out=spupng c83f765c66..., Last passed: Never

It seems that not all tests were passed completely. This is an indication that the output of some files is not as expected (but might be according to you).

Check the result page for more info.