test(pdp): establish addPieces storage baseline

[Kubuxu]

Summary

• Add a characterization test for storage used by addPieces on an existing, seeded data set.
• Record storage activity using Foundry state-diff recording.
• Report five Filecoin-relevant storage metrics:
  • storage reads
  • storage writes
  • KAMT objects touched
  • KAMT objects modified
  • newly occupied storage slots
• Group storage slots into arity-32 KAMT objects using slot >> 5.

Motivation

EVM gas usage does not map cleanly to Filecoin storage costs. This test establishes a storage-oriented baseline for the compact piece representation work in #286.

The test seeds the data set before recording so shared counters already occupy storage, then records adding four canonical pieces. This isolates steady-state piece addition from data-set creation and first-piece initialization.

Current Baseline

Metric	Value
Storage reads	21
Storage writes	28
KAMT objects touched	23
KAMT objects modified	18
Newly occupied storage slots	20

The 20 newly occupied slots correspond to five slots per piece in the current representation.

The assertions intentionally characterize the current implementation. As the compact representation is implemented, they should be updated to make the resulting storage improvement explicit.

Testing Performed

• forge test --match-contract PDPVerifierStorageTest -vv
• 1 passed, 0 failed

Scope

The current draft only adds test infrastructure and baseline assertions. It does not change contract behavior or storage layout.