fix: Reject multi-element sequences without internal captures

[zharinov]

Summary

• Add MultiElementScalarCapture diagnostic for sequences like {(a) (b)}* @items
• Reject patterns where multi-element sequence is captured without internal captures
• Previously compiled but silently captured only last element (e.g., only (b) nodes)

Why

{(identifier) (number)}* @items was semantically broken - it compiled to Node[] but only pushed number nodes, discarding identifier. This silent data loss should be a compile error.

Notes

• Added parse_cst() helper for parser tests that only validate syntax, not semantics
• Alternations with inconsistent arity (e.g., [(a) {(b) (c)}]*) also rejected