[csd113]

Release RustChan 1.7.0 by integrating the staging branch into main. This includes thumbnail delivery improvements, thread and posting lifecycle fixes, hostile-input and abuse-control hardening, theme refinements, and dependency updates.

Align the package/lockfile version and setup/download documentation with 1.7.0, and add release notes for the existing stable-release workflow. Native update minimum compatibility remains 1.6.6.

Settings operations explicitly release their lock on completion, so descriptors retained during concurrent process creation cannot delay a subsequent save or restart. A deterministic duplicate-descriptor regression failed before the fix and passes afterward; held-lease rejection remains covered.

Validation passed locally:

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings -D clippy::all -D clippy::pedantic -D clippy::nursery -D clippy::cargo
• cargo test --workspace --all-features
• cargo deny --locked check (existing dependency warnings)
• python3 -m unittest discover -s tools -p 'test_update_package.py'
• python3 tools/check_local_tooling.py
• git diff --check