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.

apps/tests/peek_expr_block.nyash

@@ -1,10 +1,10 @@
 static box Main {
   main(args) {
     local d = "1"
-    local dv = peek d {
+    local dv = match d {
       "0" => { print("found zero") 0 }
       "1" => { print("found one") 1 }
-      else => { print("other") 0 }
+      _ => { print("other") 0 }
     }
     return dv
   }

apps/tests/peek_return_value.nyash

@@ -1,11 +1,11 @@
 static box Main {
   main(args) {
-    // Use peek as an expression to produce a value
+    // Use match as an expression to produce a value
     local d = "1"
-    local v = peek d {
-      "0" => { 0 }
-      "1" => { 1 }
-      else => { 0 }
+    local v = match d {
+      "0" => 0,
+      "1" => 1,
+      _ => 0
     }
     // Show the result and finish
     local console = new ConsoleBox()