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/lsp/server.zig

@@ -1652,6 +1652,7 @@ pub const Server = struct {
             .kw_true,
             .kw_false,
             .kw_enum,
+            .kw_error,
             .kw_case,
             .kw_break,
             .kw_continue,