cmd+selfhost+test: predeclare nomem in universe scope

Per Hare convention, `nomem` is a language-level error type — no
import required, in scope alongside void/done/rune/str. ref/hare uses
it bare at errors/string.ha:14, types/c/strings.ha:89, net/uri/parse.ha:17
with no `use`. Precondition for graduating the `alloc` builtin to
`(*T | nomem)` returns.

cstage: ty_nomem is NAMED{under=ty_void, iserror=1}, installed by
typesinit and surfaced via lookup_builtin. wwstage seeds the same
shape in both check.ww (scope) and cgen.ww (aliases) — separate
tables, both consulted; without the cgen seed wwstage drops the
zero-init for `let e: nomem;` locals and breaks byte-identity.

Tests: tagged_ptr_ret.ww and trypromote.ww drop their local
`type nomem = !void;` aliases. 990_selfhost.c adds a regression that
a value named `nomem` does not collide with the predeclared type.

test/wcc/990_selfhost.c

@@ -447,6 +447,16 @@ probe_ww_compile(const char *bin)
 		  "  o.p = in_;\n"
 		  "  return o.p.val: i32;\n"
 		  "};", 42 },
+		/* #29: `nomem` predeclared in the universe scope as a TYPE
+		 * must not block a value of the same name. scope_lookup is
+		 * kind-aware, so `let nomem: i32 = 42;` shadows the type in
+		 * value position. Byte-identity vs cstage proves both stages
+		 * route the value-position lookup past the universe-scope
+		 * type seed. */
+		{ "fn main() i32 = {\n"
+		  "  let nomem: i32 = 42;\n"
+		  "  return nomem;\n"
+		  "};", 42 },
 		{ NULL, 0 },
 	};
 	char rt[1024];