fix: wdpost: fix vanilla proof indexes

[magik6k]

Related Issues

Proposed Changes

• Read proofs into an array at fixed indexes instead of appending to a list
  • Appending wasn't deterministic because challenges are read in parallel
• Then compact skipped vanilla proofs, keeping the correct order

Additional Info

• Testing:
  • Compute a PoSt with a bunch of sectors per partition (one partition is enough)
  • Compute a PoSt with skipped sectors (disable PoSt pre-checks so that this logic catches bad sectors)

Checklist

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