selfhost/cmd/wcc: collapse type-kind predicates onto n.type_ (A.6.3b, #46)

The node-keyed kind helpers (typeis8byteprimitive, isstrtype/raw,
isslicetype/raw, istaggedtype/raw, isfloattype, isf32type/raw,
isf64typeraw, isnullabletype) each re-walked TNAME aliases via
aliaslookup and peeled TBANG by hand — duplicating cstage's single-
peel kind predicates at the AST level. After A.6.2 every type-AST
kind these read is tinfo-stamped at check.ww L426-436, and
tinfofornode collapses N_TBANG (check.ww:1145-1152) and the TY_NAMED
chain, so each predicate folds to one tinfo read.

Six new tinfo helpers in lib/ww/typ.ww mirror their cstage SSoT
verbatim:

  typeisstr      — cstage cgen.c:159 `type_isstr`     (TY_STR / TY_UNTYPED_STR)
  typeisslice    — cstage cgen.c:174 `type_isslice`
  typeistagged   — cstage cgen.c:516 `type_istagged`
  typeisf32      — cstage cgen.c:188 `type_isf32`
  typeisnullable — cstage cgen.c:396 `type_isnullable` (reads tinfo.nullable
                                                       stamped at check.ww:1309-1318)
  typeis8byteprim — cstage cgen.c N_LET sz==8 ladder (slot-pad set)

Rule 9 carve-out per the A.6.3a precedent: each helper has a named
cstage counterpart; the wwstage shape mirrors it directly. The five
dead AST-walking variants (isstrtyperaw, isslicetyperaw,
istaggedtyperaw, isf32typeraw, isf64typeraw) are deleted; the five
remaining callsites (cgenstmt cglet / cgmlet str-routing, cgenexpr
cgdot tuple-field) graduate to the alias-aware isstrtype(c, t).

nullableptrtag stays AST-keyed for now — tinfofornode doesn't
populate TY_TAGGED.params (check.ww:1287-1337 sets size / align /
nullable but not the variant chain), so the tinfo equivalent of
cstage cgen.c:405 `nullable_ptr_tag` can't read params today. WHY
comment at the site cites #50 / A.6.3f as the graduation point,
alongside the variant-index work and the tparam-population glue.

Byte-identity (994/995) is the behavior gate; full `make test` green
at 133/133 confirms.

selfhost/cmd/wcc/cgenexpr.ww

@@ -1485,7 +1485,7 @@ fn cgdot(c: *cgen, n: *node) void = {
 						};
 						if (tp != nil) {
 							let tpt: *node = tp.lhs;
-							if (isstrtyperaw(tpt)) {
+							if (isstrtype(c, tpt)) {
 								emitline("\tMOVQ\t");
 								emitoff((lc.off + foff + 0): i64);
 								emitline("(BP), AX\n");