Port srchr to Rust

[bsg62]

Summary

• Replace shell-based srchr with a self-contained Rust live-grep TUI
• Embed gitignore-aware name/content search, syntect preview, and editor launch handling
• Retire shell ports and shell smoke tests after Rust parity

Test Plan

• cargo fmt --manifest-path rust/Cargo.toml -- --check
• cargo clippy --manifest-path rust/Cargo.toml -- -D warnings
• cargo test --manifest-path rust/Cargo.toml
• manual TUI smoke test confirmed by user