fix: release workspace locks held by duplicate descriptors

[adamcavendish]

Summary

• Release the workspace lock explicitly when the last Workspace owner drops.
• Prevent duplicated or inherited lock descriptors from keeping Registry::open blocked after shutdown and owner release.
• Add a Unix regression test that duplicates the lock descriptor, drops the workspace, and verifies the same workspace can be reopened.

The crate version remains 0.0.1; this PR does not publish a new crate release.

Verification

• cargo test --locked passed; all non-ignored tests passed.
• cargo build --locked passed.
• cargo clippy --locked --all-targets -- -D warnings passed.
• cargo fmt --all -- --check passed.
• git diff --check passed.

Documentation

• Added an Unreleased CHANGELOG entry describing the workspace lock lifecycle fix.
• README and Registry/HTTP lifecycle documentation remain accurate and require no changes.