feat: implement atomic versioned storage contract

[Freyskeyd]

Summary

• Add the chekov-storage versioned storage contract and memory backend.
• Validate read revisions and mutation compare-and-swap expectations before committing.
• Advance per-key revisions deterministically and preserve deletion tombstones.
• Report conflicts explicitly and prove no partial commits under stale or concurrent writers.

Validation

• cargo test --locked
• cargo clippy -p chekov-core -p chekov-storage --all-targets -- -D warnings
• cargo fmt --all -- --check

Resolves CHE-4

[linear-code]

CHE-4

[Copilot]

🟡 Changes recommended

Every commit clones the complete and continually growing state map while holding the exclusive lock.

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Pull request overview

Adds an atomic, version-aware storage contract and shared in-memory backend.

Changes:

• Introduces compare-and-swap validation and explicit conflict errors.
• Preserves tombstone revisions and prevents partial commits.
• Tests lifecycle, stale writes, concurrency, and overflow.

File summaries

File	Description
crates/chekov-storage/src/lib.rs	Implements the contract, backend, and tests.
crates/chekov-storage/Cargo.toml	Defines the new crate.
Cargo.lock	Registers the new package.

Review details

• Files reviewed: 2/3 changed files
• Comments generated: 1
• Review effort level: Balanced

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[Freyskeyd]

@codex

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Already looking forward to the next diff.

Reviewed commit: e34f4b088a

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