selfhost/cmd/wcc: enable asserttyped + close A.6.2 (#4, #15)

Final closer for the A.6.2 sequence. Mirror of harec's
`assert(expr->result)` at ref/harec/src/check.c:3810: every value-
producing nkind dispatched by resolvewalk (L475-488 + N_DOT at L391)
reaches a stamping arm in exprtype that sets e.type_ before
returning. asserttyped is the post-checker invariant gate; it walks
checkfile's decls in pass 3 (same curmod context exprtype saw in
pass 2) and writes a one-line stderr diagnostic for any dispatched
node whose type_ remained nil.

Three residual gates encode bails that aren't true gaps until #19
(Drew's δ: dedicated AST kinds for alloc/size/etc.) retires the
seeded-SK_FN-with-nil-decl + SK_USE-as-value shapes:
  1. N_IDENT resolving to SK_USE (module ref like `os` in `os.write`)
  2. N_IDENT whose sym.decl == nil (pseudo-builtin callee — len,
     append, free, alloc, size, align, offset seeded at L86-98)
  3. N_IDENT in LHS-of-N_DOT syntactic position (member-access
     lookup target, not value-producing) — tracked via `indot` param

ZERO fires across all 5 selfhost combined.ww corpora (wcc, w6c, w6a,
w6l, wwdump). asserttyped IS the regression catch — future commits
that drop a type_ stamp will fire it during the 990_selfhost probes;
no standalone table-driven test is bundled.

Accreted folds:
- 5-lite-a (#33): dispatcher-invariant docstring at exprtype L1535.
- 5-lite-b (#34): WHY comments at 9 helper bail sites
  (unifyarith/binoptype/unoptype/indexresult) classifying each as
  unreachable-for-valid-input, propagation-from-callee, or
  invalid-input (cstage errors at the matching cite). Cites
  ref/harec/src/types.c type_promote on the function-doc updates.
- A.6.2.1c (#24): three propagation pointers re-cite the
  inherent-IDENT bail at exprtype N_IDENT arm L1596-1599.
- unoptype TK_STAR dead `if (u == nil) { return nil; }` removed —
  resolvealias(unwrapbang(non-nil)) is non-nil by parser invariant
  (parsetype L148 always sets N_TBANG.lhs; resolvealias L514-569
  every exit returns non-nil for non-nil input).

lib/ww/ast.ww: nkname becomes export so asserttyped's diagnostic can
format the offending node's kind without duplicating the table.

Closes #4 (A.6.2 umbrella) and #15 (A.6.2.1e).

lib/ww/ast.ww

@@ -149,7 +149,7 @@ export fn newnode(k: nkind, file: str, line: i32, col: i32) *node = {
 
 // ---- printer ----------------------------------------------------------
 
-fn nkname(k: nkind) str = {
+export fn nkname(k: nkind) str = {
 	if (k == nkind.N_NONE)      { return "none"; };
 	if (k == nkind.N_INTLIT)    { return "int"; };
 	if (k == nkind.N_FLOATLIT)  { return "float"; };