fix(todos): serialize raw board deletion

[senamakel]

Summary

• serialize raw board deletion with the same per-thread lock as CRUD mutations
• add a deterministic regression test proving delete cannot race and resurrect an in-flight edit
• document the raw get, delete, and import_if_absent lifecycle APIs and their persistence semantics

Follow-up to #80.

API Or Behavior Changes

graph::todos::store::delete now waits for in-flight mutations on the same thread before checking and deleting the board. No signature changes.

Tests

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo clippy --all-targets --all-features -- -D warnings
• cargo build --all-targets
• cargo build --all-targets --all-features
• cargo test
• cargo test --all-features
• cargo test graph::todos::test::store_tests --lib (12 passed)

Documentation

Updated src/graph/todos/README.md and docs/modules/graph/todos.md with the raw lifecycle APIs and locking/normalization semantics.

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⚙️ Run configuration

Configuration used: Organization UI

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Run ID: 703fdc38-416c-4bb8-8149-6c1613d9c798

📥 Commits

Reviewing files that changed from the base of the PR and between ff76d76 and b6a4120.

📒 Files selected for processing (4)

• docs/modules/graph/todos.md
• src/graph/todos/README.md
• src/graph/todos/store.rs
• src/graph/todos/test.rs

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