An empty `[]` carries no element type; ww gets it only from a let
annotation (the #45 retype). Both stages used to silently default the
element to u8, and in value-form positions (return / call-arg) the
lowering miscompiled — malloc(8) ignoring n, a 16B *u8|nomem where a 24B
slice was expected (#5). Now every empty alloc that isn't a
let-annotated binding fails to infer with a loud error, aligning ww DOWN
to harec (ref/harec/src/check.c:1801-1802).
Mechanism: clet / checkletassign flags the single alloc call node that a
`let x: []T =` rescues (save/restore around the init walk); the alloc
branch errors on any empty alloc that isn't that node. The #45 wide-T
retype path is kept. wwstage needs an extra not-yet-stamped guard because
resolvewalk re-types value nodes context-free after checkletassign.
Tests: negative cstage-driver 729 (table-driven: bare-let, return,
call-arg, assignment) + positive @test in attest_pass.ww exercising the
u8 and the wide-i32 (#45) paths at runtime. Both stages reject
symmetrically; byte-id verified on []u8 and []i32.
