Make history scrolling, copy, and idle rendering quieter

[danielsuit]

Summary

• reduce mouse-wheel history movement from three rows to one
• anchor transcript selections to history so they survive scrolling and can span viewports
• stop repainting on idle poll timeouts so iTerm2 no longer sees continuous output while sc is idle
• keep active-turn animations and paste debounce responsive
• make the existing incremental-session test wait for the asynchronous writer flush instead of racing it in CI

Validation

• cargo test --workspace --target-dir /tmp/subconscious-target-history-copy
• cargo clippy --workspace --all-targets --target-dir /tmp/subconscious-target-history-copy -- -D warnings
• cargo fmt --all -- --check