perf: PDF generation clones every element string into the group vec

[stultus]

Severity: low
Location: src-tauri/src/screenplay/pdf.rs:116-192 (group_elements and surroundings)

group_elements() clones element_type, text, and typst_inline for every element. For a 120-scene screenplay with ~2000 nodes, that duplicates several megabytes of string data during PDF generation.

This isn't on the editor hot path (export is a one-shot operation) but it does show up in export latency for long scripts.

Fix: Take the input by &[Element] and emit groups holding references with explicit lifetimes; or, since elements aren't reused after grouping, swap clone() for mem::take() / move semantics on the source vec.