ERR/E0.1: error-set decls + ! / !Named type exprs (parser)

Parser-only first step of the error-handling stream. No sema/codegen.

- token: `kw_error` keyword (`!` reuses existing `.bang`).
- ast: `ErrorSetDecl { name, tag_names }` + `ErrorTypeExpr { name: ?[] }`
  (null = inferred `!`, non-null = `!Named`); wired into Node.Data and
  declName.
- parser: `parseErrorSetDecl` (comma-separated tags, optional trailing
  comma/`;`) dispatched from parseConstBinding; `!` / `!Named` parsed in
  parseTypeExpr; result-list loop enforces error type as trailing-only;
  hasFnBodyAfterArrow skips `.bang` so failable-return fns are recognised.
- print: new focused AST round-trip printer (decls + type exprs); loud
  `error.UnsupportedNode` otherwise. Registered in root.zig.
- sema/lsp: exhaustive switch arms for the two new nodes.
- tests: 11 inline parser unit tests (shapes + 3 round-trip prints + 2
  trailing-position rejections).

zig build, zig build test, and 254/254 examples green.

src/ast.zig

@@ -37,6 +37,7 @@ pub const Node = struct {
         struct_decl: StructDecl,
         struct_literal: StructLiteral,
         union_decl: UnionDecl,
+        error_set_decl: ErrorSetDecl,
         lambda: Lambda,
         type_expr: TypeExpr,
         param: Param,
@@ -56,6 +57,7 @@ pub const Node = struct {
         pointer_type_expr: PointerTypeExpr,
         many_pointer_type_expr: ManyPointerTypeExpr,
         optional_type_expr: OptionalTypeExpr,
+        error_type_expr: ErrorTypeExpr,
         pack_index_type_expr: PackIndexTypeExpr,
         comptime_pack_ref: ComptimePackRef,
         force_unwrap: ForceUnwrap,
@@ -94,6 +96,7 @@ pub const Node = struct {
                 .enum_decl => |d| d.name,
                 .struct_decl => |d| d.name,
                 .union_decl => |d| d.name,
+                .error_set_decl => |d| d.name,
                 .namespace_decl => |d| d.name,
                 .ufcs_alias => |d| d.name,
                 .c_import_decl => |d| d.name,
@@ -324,6 +327,13 @@ pub const UnionDecl = struct {
     field_types: []const *Node,
 };
 
+/// `Foo :: error { TagA, TagB }` — a named error set. Tags are bare
+/// identifiers (no payload, no explicit value), unlike enum variants.
+pub const ErrorSetDecl = struct {
+    name: []const u8,
+    tag_names: []const []const u8,
+};
+
 pub const StructTypeParam = struct {
     name: []const u8, // e.g. "N" or "T" (without $)
     constraint: *Node, // type_expr: "u32" for value param, "Type" for type param
@@ -461,6 +471,15 @@ pub const OptionalTypeExpr = struct {
     inner_type: *Node,
 };
 
+/// The error channel of a multi-return result list: bare `!` (inferred
+/// set) or `!Named` (a declared `error { ... }` set). Appears only as
+/// the trailing result element; the parser enforces the position and
+/// sema (E1) restricts it to return positions.
+pub const ErrorTypeExpr = struct {
+    /// `null` = inferred set (bare `!`); non-null = named set (`!Named`).
+    name: ?[]const u8 = null,
+};
+
 pub const ForceUnwrap = struct {
     operand: *Node,
 };