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.
Per CLAUDE.md rule 10, align cstage down to wwstage — when every
variant in a `?` propagation maps to itself, the remap loop emits
zero JMPs and the propret label is dead. Lazy-allocate it so the
label-counter ID is only consumed when at least one JMP fires.
Smoke test selfhost/test/trypromote.ww exercises same-shape
(i64|nomem)→(i64|nomem) propagation; cstage and wwstage now emit
byte-identical asm for the TRYPROP region.
