Added lastVerifiedBalanceTimestamp to CompoundingStakingStrategy

[naddison36]

Summary

Adds lastVerifiedBalanceTimestamp to CompoundingStakingStrategy so integrations can determine when beacon-chain balance data was last successfully verified.

The value records the timestamp captured by snapBalances(), not the later transaction timestamp of verifyBalances().

Changes

• Add the public uint64 lastVerifiedBalanceTimestamp storage variable.
• Update it only after all validator and pending-deposit proofs pass in verifyBalances().
• Leave it unchanged when:
  • snapBalances() is called without a successful verification.
  • verifyBalances() reverts.
• Expose the getter through ICompoundingStakingStrategy.
• Document the new accounting behavior.

Existing deployments will return 0 until their first successful balance verification after upgrading. No migration or backfill is required.

Storage layout

The new variable consumes one reserved storage slot:

• lastVerifiedBalanceTimestamp is appended at slot 61.
• __gap is reduced from 40 to 39 slots.
• The gap still ends at slot 101, so downstream inherited storage is unchanged.
• The pinned storage-layout test has been updated accordingly.

Tests

Added unit coverage confirming:

• The initial value is zero.
• snapBalances() alone does not update it.
• A successful verification stores the snapshot timestamp rather than the verification transaction timestamp.
• Clearing snappedBalance.timestamp does not clear the last verified timestamp.
• A failed verification preserves the previous value.
• A later successful verification advances the value.

Verification performed:

• make build-contracts
• Focused Compounding Staking Strategy tests: 28 passed
• pnpm test:layouts: 11 passed
• git diff --check

Code Change Checklist

• Contract code is complete
• Executable deployment file — not included in this PR
• Fork tests after deployment — not applicable without a deployment file
• Unit tests
• Owner completed the full contract code review checklist
• Two approvals by internal reviewers

[sparrowDom]

Great thanks