feat: harden DataStore ownership lock

[enricopiovesan]

Summary

• harden local DataStore lock failures so they never expose host roots
• document the standard-library lock lifecycle and deterministic unsupported-platform behavior
• prove cross-process contention, clean owner release, and owner-crash recovery

Governing Spec

• 518-durable-local-datastore
• 519-embedder-owned-datastore-integration

Project Item

• Harden cross-platform local DataStore ownership locking #825 Harden cross-platform local DataStore ownership locking

Definition of Done

• Stable store_locked details never disclose lock paths or roots.
• Cross-process contention preserves committed state and reports store_locked.
• Owner release and process-termination recovery permit a later reopen.
• Unsupported locking fails with a deterministic typed error.
• No lease protocol or multi-process write policy was added.

Validation

• cargo clippy -p traverse-runtime --lib -- -D warnings
• cargo test -p traverse-runtime data_store --lib
• cargo test --workspace
• bash scripts/ci/spec_alignment_check.sh /private/tmp/traverse-pr-825.md
• cargo fmt --check
• git diff --check