perf(tui): share preview ownership

[gitkeniwo]

Summary

• Share one catalog snapshot between the TUI and search index.
• Borrow live previews and reference-count trash previews instead of copying snippet content per frame.
• Add allocation regression coverage and record the completed work in the changelog and roadmap.

Verification

• cargo fmt --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo clippy --locked --all-targets --no-default-features -- -D warnings
• cargo test --locked --all-features