Fix GLR panic on optional empty branch

[ehwan]

close #89

Nadrieril#1

GLR parser panicked with assertion failed: self.node(node).is_leaf() when a grammar rule contained an optional branch (A B)? and the same lookahead token triggered both a shift and an ε-reduce simultaneously.

Root cause

In feed_location_impl, the prevent_leaf mechanism bumps node.child_count to prevent premature freeing during multi-reduce processing. When prepare_reduce_node is called with count=0 (empty rule) on a non-leaf node, it takes the i == node.len() path and calls add_child(node, new_child), incrementing child_count a second time. After the reduce loop restores child_count by 1, the node still has child_count=1. The shift code then mutates and pushes this non-leaf node to next_nodes, breaking the is_leaf() precondition on the next feed call.

Fix

• rusty_lr_core/src/parser/nondeterministic/context.rs: Before performing the shift after the reduce loop, check is_leaf(). If the node has acquired children during reduce processing, allocate a fresh leaf child to carry the shift result — mirroring how prepare_reduce_node already handles the reduce branches. The original node becomes a shared prefix for both paths.
• example/glr/src/main.rs: Three regression tests reproducing the grammar from the issue (amp self_kw, amp bool_kw, ident colon amp bool_kw).