feat: add lightweight state checksum checkpoints

[bmuddha]

What changed

Signed checksum checkpoints detect persisted-state divergence between full superblocks without creating snapshots, archiving state, or rotating the ledger. Producers sample fresh state at an ordered execution boundary; followers and local replay stop on a mismatch.

Crate READMEs focus on purpose and integration contracts rather than implementation details.

Closes #154

Impact

• BlockstoreParams::checkpoint sets an independent slot interval and defaults to disabled. Full superblocks replace checkpoints when both are due; followers consume upstream records regardless of local settings.
• Checkpoints scan persisted accounts while execution is paused and use ordinary buffered ledger publication. They add no forced disk sync; full superblocks remain the recovery boundaries.
• Wire and storage version numbers remain unchanged for the MVP. Checkpoint streams require checkpoint-aware peers; no migration or mixed-version support is added.

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📝 Walkthrough

Walkthrough

The change adds configurable signed checksum checkpoints. AccountsDB can compute a fresh persisted-state checksum without flushing or updating its cached value. Ledger schemas, appenders, readers, and replication now support checkpoint records. The pacemaker emits checkpoints independently from superblocks. Replay and followers verify checkpoint payloads and stop on mismatches. Tests cover scheduling, persistence, recovery, replication, and signature validation. Several README files were condensed or rewritten.

Priority: ➖ Normal

Change: Feature

Merge Risk: 🔵 Low · up to 1eed9

Fresh-checksum documentation should clearly state that writes and metadata updates must be quiesced. This is a bounded documentation correction and does not indicate a checkpoint-flow failure.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Out of Scope Changes check	⚠️ Warning	The pull request includes unrelated documentation rewrites. Examples include programs/magic-root-interface/README.md, programs/magic-root-program/README.md, `programs/v42-calculator-interface/READ…	Remove the unrelated README rewrites, or link each change to a requirement with a concrete checkpoint implementation dependency. Retain documentation changes that describe checkpoint behavior or the affected ledger, engine, keeper, accounts…

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: adding lightweight state checksum checkpoints.
Description check	✅ Passed	The description directly explains the checksum checkpoint behavior, configuration, replication, replay verification, persistence impact, and compatibility scope.
Linked Issues check	✅ Passed	Issue #154 coding requirements are implemented. Checkpoint(u64) adds a signed checksum-only ledger record with a distinct signing domain. BlockstoreParams::checkpoint provides an independent inter…
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 39 functions across 21 files. (16 skipped:…

Full details: Out of Scope Changes check

Explanation

The pull request includes unrelated documentation rewrites. Examples include programs/magic-root-interface/README.md, programs/magic-root-program/README.md, programs/v42-calculator-interface/README.md, programs/v42-calculator-program/README.md, solana/account/README.md, solana/program-runtime/README.md, solana/svm/README.md, solana/transaction-context/README.md, and solana/transaction-view/README.md. These changes describe program, account, runtime, or transaction behavior and have no concrete connection to checksum checkpoints in issue #154.

Resolution

Remove the unrelated README rewrites, or link each change to a requirement with a concrete checkpoint implementation dependency. Retain documentation changes that describe checkpoint behavior or the affected ledger, engine, keeper, accountsdb, and replicator interfaces.

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
In `@accountsdb/README.md`:
- Around line 37-38: Update the README text describing fresh sampling to state
unambiguously that writes and metadata updates must be quiesced, matching the
requirement of AccountsDB::compute_checksum; preserve the surrounding
explanation about storage flushing and cached values.

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 9669f69 and 1eed9b7.

📒 Files selected for processing (37)

• accountsdb/README.md
• accountsdb/src/lib.rs
• accountsdb/src/store/mod.rs
• accountsdb/src/tests.rs
• engine/README.md
• engine/src/error.rs
• engine/src/lib.rs
• engine/src/pacemaker.rs
• engine/tests/checkpoints.rs
• engine/tests/recovery.rs
• keeper/README.md
• keeper/src/error.rs
• keeper/src/lib.rs
• keeper/src/testkit.rs
• ledger/README.md
• ledger/src/appender.rs
• ledger/src/reader.rs
• ledger/src/schema.rs
• ledger/src/tests/integration.rs
• nucleus/README.md
• nucleus/src/config.rs
• nucleus/src/ledger.rs
• processor/README.md
• programs/magic-root-interface/README.md
• programs/magic-root-program/README.md
• programs/v42-calculator-interface/README.md
• programs/v42-calculator-program/README.md
• replicator/README.md
• replicator/src/client.rs
• replicator/src/lib.rs
• replicator/src/tests.rs
• replicator/tests/integration.rs
• solana/account/README.md
• solana/program-runtime/README.md
• solana/svm/README.md
• solana/transaction-context/README.md
• solana/transaction-view/README.md

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[bmuddha]

Regarding the README scope warning:

The crate-wide README simplification is intentional additional scope, isolated in commit 1eed9b7 and described in the PR body. It reduces implementation narration while preserving integration contracts; it is not a dependency of #154. We’re retaining it.