Fix GLR nullable reduce self-loops

[ehwan]

close #91

• Expand safe auto-generated P? occurrences when their empty branch would create a zero-consuming GLR reduce self-loop.
• Emit glr_optional_expanded info diagnostics showing the original production and generated replacement productions.
• Reject remaining zero-consuming GLR nullable cycles at generation time with actionable errors for observed P?, P*, and user-defined nullable productions.
• Wire the new error through CLI/buildscript, derive macro, and LSP diagnostics.
• Document the GLR nullable-cycle normalization/error behavior and add regression tests.

[gemini-code-assist]

Code Review

This pull request introduces automatic expansion of safe, unobserved optional (P?) occurrences in GLR grammars to prevent zero-consuming reduce cycles, and reports unexpandable nullable cycles as generation errors. The changes span the build script, derive macro, LSP diagnostics, and parser. The review comments suggest two valuable optimizations: first, batching the expansion of optional helpers in expand_glr_epsilon_self_loop_optionals to avoid repeatedly rebuilding the state machine; second, deduplicating the error formatting logic for ParseError::GlrNullableReduceCycle in the build script.

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[gemini-code-assist]

The current implementation of expand_glr_epsilon_self_loop_optionals runs a loop up to self.nonterminals.len() times, and in each iteration, it builds the entire LR/LALR state machine just to find and expand a single optional helper. For larger grammars, building the state machine is a very expensive operation.\n\nWe can significantly optimize this by collecting all candidate optional helpers that participate in a self-loop in the current state machine, and expanding all of them in a single pass before rebuilding the state machine. This reduces the number of state machine builds from O(K) (where K is the number of self-looping optionals) to O(1) in almost all cases.

            let mut rewrite_targets = BTreeSet::new();\n            for (state_idx, state) in states.iter().enumerate() {\n                for reduce_rules in state.reduce_map.values() {\n                    for &rule_idx in reduce_rules {\n                        let Some((nonterm_idx, local_rule_idx)) = self.rule_id_to_indices(rule_idx)\n                        else {\n                            continue\n                        };\n                        if !self\n                            .is_optional_epsilon_self_loop_candidate(nonterm_idx, local_rule_idx)\n                        {\n                            continue\n                        }\n                        if state\n                            .shift_goto_map_nonterm\n                            .iter\n                            .any(|(nonterm, target)| {\n                                *nonterm == nonterm_idx && *target == state_idx\n                            })\n                        {\n                            rewrite_targets.insert(nonterm_idx);\n                        }\n                    }\n                }\n            }\n\n            if rewrite_targets.is_empty() {\n                break;\n            }\n            let mut changed_any = false;\n            for optional_idx in rewrite_targets {\n                if self.expand_optional_occurrences(optional_idx) {\n                    changed_any = true;\n                }\n            }\n            if !changed_any {\n                break;\n            }

[gemini-code-assist]

This error formatting block for ParseError::GlrNullableReduceCycle is identical to the one defined at lines 892-912. To improve maintainability and avoid code duplication, consider extracting this formatting logic into a helper function or method on Builder (e.g., fn format_glr_nullable_cycle_error(...) -> Diagnostic<usize>).