Add details on target group encoding and subgroup check to SIMD-0388

[samkim-crypto]

I added additional details on:

• The target group encoding
• The fact that subgroup check should be skipped for addition and subtraction

cc: @0x0ece, @zz-sol, @Rexicon226

[simd-bot]

Hello samkim-crypto! Welcome to the SIMD process. By opening this PR you are affirming that your SIMD has been thoroughly discussed and vetted in the SIMD discussion section. The SIMD PR section should only be used to submit a final technical specification for review. If your design / idea still needs discussion, please close this PR and create a new discussion here.

This PR requires the following approvals before it can be merged:

• At least one approval from a Anza team member: t-nelson, sakridge, or bw-solana
• At least one approval from an Firedancer team member: ptaffet-jump, topointon-jump, 0x0ece, lidatong, or ripatel-fd

Once all requirements are met, you can merge this PR by commenting /merge.

[Rexicon226]

Just wondering if we should include a small example of an encoding of a GT, in both BE and LE.

[simd-bot]

Thanks, 0x0ece!

⚠️ Status: Cannot merge yet

• Missing approval from: Anza (t-nelson, sakridge, or bw-solana)