feat: add transactional journal semantics

[Freyskeyd]

Summary

Implements Linear issue CHE-2 with a runtime-independent chekov-journal layer above chekov-core.

• Adds read-your-writes overlays and nested affine checkpoints.
• Merges committed child writes and discards reverted writes.
• Retains observations from reverted scopes.
• Coalesces repeated writes into final mutations.
• Produces deterministic transition batches while sharing values through Arc.
• Adds semantic coverage for nested commit/revert, tombstones, read propagation, coalescing, and deterministic output.

Verification

• cargo test --workspace
• cargo check --workspace
• cargo fmt --all -- --check
• cargo clippy -p chekov-journal --all-targets -- -D warnings

[linear-code]

CHE-2

[Copilot]

🟡 Changes recommended

Failed checkpoint operations consume indispensable tokens, leaving open frames impossible to close.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds a runtime-independent transactional journal layer over chekov-core.

Changes:

• Implements nested checkpoints with commit/revert semantics.
• Adds read tracking, write coalescing, tombstones, and deterministic transitions.
• Adds semantic unit tests and workspace registration.

File summaries

File	Description
crates/chekov-journal/src/lib.rs	Implements journal behavior and tests.
crates/chekov-journal/Cargo.toml	Defines the new crate.
Cargo.toml	Registers workspace dependencies.
Cargo.lock	Records the new package.

Review details

• Files reviewed: 3/4 changed files
• Comments generated: 1
• Review effort level: Balanced

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[Copilot]

🔵 Needs a closer look

The transition methods return a checkpoint error variant whose documented meaning does not match the failure condition.

Review details

Suppressed comments (1)

Previously missed (1) — in code that hasn't changed since the last review.

crates/chekov-journal/src/lib.rs:78

• NotTopCheckpoint is also returned by transition and into_transition when any checkpoint remains open, although those operations receive no checkpoint token and the documented condition is therefore false. This conflates an out-of-order close with an unfinished transaction for API consumers; add a distinct OpenCheckpoints variant (and broaden the enum-level documentation) for both transition paths.

• Files reviewed: 3/4 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[Freyskeyd]

@codex

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🚀

Reviewed commit: 723b9032c6

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