feat: add checksum computation to AccountsDb snapshots

[bmuddha]

Problem:
Need a way to verify that standby nodes have replicated the exact same state as the primary. Currently AccountsDb has no mechanism to compute a hash of its entire state.

Solution:
checksum() method on AccountsDb that iterates all accounts in key-sorted order (via LMDB) and hashes both pubkey and serialized account data using xxHash3. Returns a 64-bit hash suitable for verifying state consistency across nodes.

Implementation details:

• AccountsDb::checksum() acquires write lock for consistency
• Uses xxHash3 (fast, non-cryptographic hash with good distribution)
• Hashes: pubkey (32 bytes) + serialized account data
• Iteration order is deterministic (LMDB key-sorted)

Acceptance criteria:

• New method on AccountsDb to compute state checksum
• Scans all active accounts and hashes them cumulatively
• Returns single hash value (u64)
• Integrates with existing snapshot flow (deferred)
• Unit tests for correctness and determinism

Reference: MIMD-0019 SuperBlock validation

Parent epic: #935