refactor: split optimizer/verifier/parser modules (mainline); add runner trace/directives; add LLVM terminator/select scaffolds; extract AST Span; update CURRENT_TASK with remaining plan

src/parser/expr/coalesce.rs

@@ -0,0 +1,33 @@
+use crate::parser::{NyashParser, ParseError};
+use crate::parser::common::ParserUtils;
+use crate::tokenizer::TokenType;
+use crate::ast::{ASTNode, Span};
+
+#[inline]
+fn is_sugar_enabled() -> bool { crate::parser::sugar_gate::is_enabled() }
+
+impl NyashParser {
+    pub(crate) fn expr_parse_coalesce(&mut self) -> Result<ASTNode, ParseError> {
+        let mut expr = self.expr_parse_or()?;
+        while self.match_token(&TokenType::QmarkQmark) {
+            if !is_sugar_enabled() {
+                let line = self.current_token().line;
+                return Err(ParseError::UnexpectedToken {
+                    found: self.current_token().token_type.clone(),
+                    expected: "enable NYASH_SYNTAX_SUGAR_LEVEL=basic|full for '??'".to_string(),
+                    line,
+                });
+            }
+            self.advance();
+            let rhs = self.expr_parse_or()?;
+            let scr = expr;
+            expr = ASTNode::PeekExpr {
+                scrutinee: Box::new(scr.clone()),
+                arms: vec![(crate::ast::LiteralValue::Null, rhs)],
+                else_expr: Box::new(scr),
+                span: Span::unknown(),
+            };
+        }
+        Ok(expr)
+    }
+}