test: expand persistence and board normalization coverage

[UberMetroid]

Summary

• cover task validation, version conflict, atomic replacement, legacy and corrupt storage behavior
• add board normalizer tests for defaults, unknown columns, idempotence, and save-needed behavior

Verification

• git diff --check passed
• cargo check --workspace, cargo test --workspace, and cargo clippy --workspace --all-targets -- -D warnings were attempted but the environment hit disk quota
• cargo fmt --all -- --check was unavailable because rustfmt is not installed

No cargo-deny/audit gate was added; the existing CI contract and smoke tests were otherwise left unchanged.