Language: for i in start..end loops

Adds a \`for NAME in START..END { BODY }\` half-open range loop:

    for i in 0..8 {
        total += arr[i]
    }

- Lexer: \`for\`, \`in\` keywords and the \`..\` range operator
- AST: new \`Statement::For\` variant with var/start/end/body
- Parser: \`parse_for\` after \`while\` / \`loop\`
- Analyzer: registers the loop variable as a u8 symbol for the body
  (restoring any shadowed outer symbol afterwards), allocates it via
  the normal RAM allocator, and tracks it as "used"
- IR lowering: desugars to \`var = start; while var < end { body;
  var = var + 1 }\` using a \`for_step\` continue-edge block so
  \`continue\` properly increments the index
- AST codegen: no-op (legacy path doesn't need for loops)
- Tests: parse + full-pipeline integration

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

tests/integration_test.rs

@@ -178,6 +178,25 @@ fn program_with_on_scanline_per_state() {
     rom::validate_ines(&rom_data).expect("should be valid iNES");
 }
 
+#[test]
+fn program_with_for_loop() {
+    let source = r#"
+        game "ForLoop" { mapper: NROM }
+        var arr: u8[8] = [0, 0, 0, 0, 0, 0, 0, 0]
+        var total: u8 = 0
+        on frame {
+            total = 0
+            for i in 0..8 {
+                total += arr[i]
+            }
+            wait_frame
+        }
+        start Main
+    "#;
+    let rom_data = compile(source);
+    rom::validate_ines(&rom_data).expect("should be valid iNES");
+}
+
 #[test]
 fn program_with_structs() {
     let source = r#"