feat: persist caller-managed chain slot in accountsdb

[bmuddha]

What changed

Callers can now retain an external chain slot in AccountsDB through accounts().chain_slot() and accounts().set_chain_slot(slot), independently of local execution progress.

Closes #169

Impact

The slot starts at zero and accepts rewinds. Updates don't flush on their own; existing sync and snapshot operations persist the metadata. The slot is excluded from execution checksums.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository: magicblock-labs/magicblock-engine/.coderabbit.yaml

Review profile: ASSERTIVE

Plan: Advanced

Run ID: 45f4d11d-070a-40f2-92db-6ef645d3bad7

📥 Commits

Reviewing files that changed from the base of the PR and between cd08685 and e78dff9.

📒 Files selected for processing (3)

• accountsdb/README.md
• accountsdb/src/lib.rs
• accountsdb/src/store/mmap.rs

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📝 Walkthrough

Walkthrough

DatabaseMeta adds an atomic for the caller-managed chain slot. AccountsDB adds methods to read and set that value. The setter accepts values lower than the current slot. The README adds the chain slot to the checksum exclusions.

Priority: ⬇️ Low

Change: Feature

Merge Risk: ⚪ Minimal · up to e78df

The caller-managed chain slot is independent of execution progress and checksums and persists through existing flush and snapshot operations. No actionable merge-blocking risk was identified.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Issue #169 requires a caller-managed slot through Keeper's accounts accessor, rewind support, metadata persistence, and checksum independence. AccountsAccessor dereferences to AccountsDB, so `acco…
Out of Scope Changes check	✅ Passed	The changes add the requested AccountsDB metadata and accessor methods and update checksum documentation for issue #169. No unrelated product behavior or unrelated files are shown in the supplied chan…
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 2 files. (1 skipped: 1 …
Title check	✅ Passed	The title clearly and concisely describes the primary change: persisting a caller-managed chain slot in AccountsDB.
Description check	✅ Passed	The description directly explains the new AccountsDB APIs, persistence behavior, rewind support, and checksum exclusion.

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