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/token.zig

@@ -13,6 +13,7 @@ pub const Tag = enum {
     kw_true,
     kw_false,
     kw_enum,
+    kw_error, // error (error-set declaration)
     kw_case,
     kw_break,
     kw_continue,
@@ -222,6 +223,7 @@ pub const keywords = std.StaticStringMap(Tag).initComptime(.{
     .{ "true", .kw_true },
     .{ "false", .kw_false },
     .{ "enum", .kw_enum },
+    .{ "error", .kw_error },
     .{ "case", .kw_case },
     .{ "break", .kw_break },
     .{ "continue", .kw_continue },