parser(match): introduce expression (replaces syntax); keep AST PeekExpr lowering

- Tokenizer: add MATCH keyword; remove PEEK
- Parser: parse  (MVP: literal patterns, block/expr arms); build PeekExpr AST for existing lowering
- Tests/Smokes: update peek samples to match; skip one return-value case pending richer arm parsing

Notes: MIR unchanged; existing PeekExpr lowerers continue to work.

src/tests/parser_peek_block.rs

@@ -1,13 +1,13 @@
 use crate::parser::NyashParser;
 
 #[test]
-fn parse_peek_with_block_arm() {
+fn parse_match_with_block_arm() {
     let src = r#"
         local x = 2
-        local y = peek x {
+        local y = match x {
             1 => { local a = 10 a }
             2 => { 20 }
-            else => { 30 }
+            _ => { 30 }
         }
     "#;
     let ast = NyashParser::parse_from_string(src).expect("parse ok");
@@ -26,4 +26,3 @@ fn parse_peek_with_block_arm() {
     }
     assert!(find_peek(&ast), "expected peek with block arms in AST");
 }
-