fix(avc): Remove unnecessary TODO for idr_pic_id

[cfsmp3]

Summary

Removes the TODO comment at src/lib_ccx/avc_functions.c:997 and adds a clarifying comment explaining why idr_pic_id is read but not stored.

Why idr_pic_id doesn't need to be stored:

1. Bitstream parsing requirement: The value must be read to advance the bitstream position - subsequent fields like pic_order_cnt_lsb come after it in the slice header

2. Not needed for caption extraction: CCExtractor uses pic_order_cnt_lsb for frame ordering (when --usepicorder is set) and PTS for timing synchronization - neither requires idr_pic_id

3. Purpose in H.264: idr_pic_id distinguishes between consecutive IDR pictures for decoder error recovery - not relevant for extracting embedded captions from SEI NAL units

Closes #1895

🤖 Generated with Claude Code

[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 6dcdb4b...:

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	86/86
Teletext	21/21
WTV	13/13
XDS	34/34

Your PR breaks these cases:

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

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.