chore: fix all clippy warnings; remove dead frag_end variable
- tala-format: collapse 4 nested if chains into && let chains
- tala-typst: loop+match -> while let; suppress too_many_arguments on
frame walkers (context struct adds indirection without clarity gain);
remove dead frag_end variable and its let _ = frag_end suppress hack
- tala: &PathBuf -> &Path on persist_dir and render_preview; add Path
import; 4 redundant || Vec::new() closures -> Vec::new fn pointers
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>