Use correct start index for Art-Net in proxy mode

[askask]

Art-Net data doesn't contain a start byte. This is currently only taken into account when using regular LED output.
With this commit it also works when using DMX proxy mode.

Summary by CodeRabbit

• Bug Fixes
  • Fixed DMX data alignment across Art-Net and E1.31 lighting protocols for proper channel synchronization and correct data handling when using multiple protocol paths.

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[softhack007]

@troyhacks, @netmindz: If I remember correctly, you are regularly working with Art-Net? Would like to get your thoughts on this PR.

[troyhacks]

I think I understand it - it's just aligning incoming Art-Net/E1.31 data so channels from an Art-Net packet hits DMX output on the correct channel.

I haven't used this feature, but I think it's safe to merge. Good catch, @askask!

[github-actions]

Hey! This pull request has been open for quite some time without any new comments now. It will be closed automatically in a week if no further activity occurs.
Thank you for contributing to WLED! ❤️

[netmindz]

I'm not sure WLED is really appropriate for trying to be a an ArtNet/sACN to DMX (wired) convertor, but if has that feature it should certainly map the right values to the right channel.

If we are looking for confirmation of fix before merge with actual hardware I can probably rustle something up.

For future reference I would always recommend using a dedicated esp32 if you want such bridge from wireless to wired DMX, lighting desk send a universe at up to 44 Hz which WLED can't provide so you will lose frames and moving heads might behave erratically when getting a slow and inconsistent update

[softhack007]

@coderabbitai review

[coderabbitai]

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[coderabbitai]

Walkthrough

Introduces an artnetOffset variable to unify DMX data indexing between Art-Net and E1.31 protocols in e131.cpp. The DMX write logic now applies this offset during data access, and dataOffset adjustment is consolidated into a single calculation path instead of conditional branching.

Changes

Cohort / File(s)	Summary
DMX Protocol Offset Handling wled00/e131.cpp	Introduces artnetOffset variable to normalize indexing differences between Art-Net (offset −1) and E1.31 (offset 0). Modifies handleE131Packet to apply offset during DMX data read operations and consolidates dataOffset adjustment logic, removing separate Art-Net conditional decrement.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: correcting Art-Net start index handling in proxy mode to align incoming data with DMX output channels.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

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