[PVM-host] Fix accumulation pipeline and add arithmetic overflow guards

[0xjunha]

Summary

1. Fix accumulation pipeline

• Preimage requests for removed/missing accounts should be ignored.
• Provided preimages should not be integrated if the request is dropped.
• Storage / Preimages / Lookups entries should not be integrated in Service Account STFs if account metadata is removed.
• Add deferred transfer gas limits from previous rounds in the outer accumulation (Δ+) for processable work reports count estimation.

2. Add overflow guards

• Use checked_* methods for safe arithmetics with proper error handling.
• Add guard for type conversions from register values in host functions.