feat: add optimized accountsdb crate

[bmuddha]

What changed

Added the accountsdb crate with persisted and volatile stores, routing logic, snapshot support, and mmap/LMDB-backed storage.

Why

The engine needs one account database that routes mutable and immutable accounts to the correct backend and removes stale copies when an account changes mode.

Closes #6.

Impact

• Introduces AccountsDB, loaders, iterators, snapshots, backups, and error handling.
• Adds persisted storage, volatile storage, and the on-disk index layout.
• Keeps persisted offsets measured in StorageUnit and converts to bytes only at mmap/file boundaries.

Reviewer notes

Snapshot creation requires exclusive write access. The persisted layout and storage-unit arithmetic are the sharp edges to review closely.

Follow-up

ledger and keeper consume this API upstack.

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[bmuddha]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat: adds top level engine crate for global orchestration #24
• feat: added internal built-in magic root program #23
• feat: implement transaction processor crate #22
• feat: implements keeper crate - state orchestrator #17
• feat: implement superblock based ledger crate #15
• feat: add optimized accountsdb crate #14 👈 (View in Graphite)
• feat: add nucleus engine crate #18
• feat: add svm crate #21
• feat: add program runtime crate #20
• feat: add transaction context crate #19
• feat: add customized solana-account crate #13
• master

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