Fix left navigation from file mode sections

[gaoflow]

Summary

• make left navigation collapse only expandable changed sections
• let left navigation move directly from file mode sections back to their file
• add a regression snapshot for the FileMode-only path

Fixes #100

Tests

• cargo fmt --check
• cargo test -p scm-record
• cargo clippy --workspace --all-features --all-targets -- --deny warnings
• cargo test --all-features --examples --tests --workspace
• cargo test --all-features --doc --workspace
• cargo test --no-default-features --examples --tests --workspace
• cargo test --no-default-features --doc --workspace