Language: struct types

Adds composite \`struct\` types with field access:

    struct Vec2 { x: u8, y: u8 }

    var pos: Vec2
    pos.x = 100
    pos.y = pos.x + 5

- Lexer: \`struct\` keyword
- AST: \`StructDecl\` with \`StructField\` list; \`NesType::Struct(name)\`
  for struct-typed variable declarations; \`Expr::FieldAccess\` and
  \`LValue::Field\` for reads/writes
- Parser: top-level \`struct Name { field: type, ... }\` (optional
  trailing comma) and \`ident.field\` syntax in both expression and
  lvalue position
- Analyzer: \`register_struct\` computes contiguous field offsets
  (no padding) and stores them in \`struct_layouts\`. Struct variables
  synthesize a \`VarAllocation\` per field under the name
  \`"struct_var.field"\`, and \`Expr::FieldAccess\` / \`LValue::Field\`
  resolve against those. Unknown struct types and unknown fields
  emit E0201.
- IR lowering + AST codegen: treat struct field access as ordinary
  variable access against the synthetic per-field symbols. No new IR
  ops are needed.

v1 structs only support primitive fields (u8/i8/bool). Nested structs,
u16 fields, and array fields are not yet allowed.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

tests/integration_test.rs

@@ -141,6 +141,29 @@ fn program_with_functions() {
     rom::validate_ines(&rom_data).expect("should be valid iNES");
 }
 
+#[test]
+fn program_with_structs() {
+    let source = r#"
+        game "Structs" { mapper: NROM }
+        struct Vec2 { x: u8, y: u8 }
+        struct Player { health: u8, lives: u8 }
+
+        var pos: Vec2
+        var hero: Player
+
+        on frame {
+            pos.x = 100
+            pos.y = 50
+            hero.health = 3
+            hero.lives = 5
+            if button.right { pos.x += 1 }
+        }
+        start Main
+    "#;
+    let rom_data = compile(source);
+    rom::validate_ines(&rom_data).expect("should be valid iNES");
+}
+
 #[test]
 fn program_with_enums() {
     let source = r#"