selfhost: port match exhaustiveness, ?-subset, !-flag checks to check.ww

The selfhost checker did name resolution only — anything tagged-
union-shaped sailed through silently. The C check.c implements
three structural checks; this commit mirrors them at the AST level
in selfhost/cmd/wcc/check.ww:

1. Match exhaustiveness: every variant of the scrutinee's tagged
   union must be covered by a case arm (incl. multi-pattern alts)
   or a default arm. Operates on the scrutinee's declared type
   (N_TTAGGED via N_IDENT's sym.decl.lhs).

2. ? subset propagation: each error variant of the operand's type
   must be a variant of the enclosing fn's return type. Enclosing
   return must itself be a tagged union when the operand has any
   errors.

3. !-flag semantics: in flag-aware mode (any variant marked `!T`),
   error subset = flagged variants. Legacy mode (no flags) =
   everything-but-first. is_error_variant unifies both rules.

No tinfo / type-inference work: the checks read declared AST type
nodes directly. `resolvealias` chases N_TNAME → typedecl body to
handle aliased tagged unions. `type_eq_ast` does structural
comparison on the subset of type-expression shapes the checks
encounter (TNAME by string, TPTR/TSLICE/TCHAN recursive).

Folded into resolvewalk rather than a separate second pass, so the
checks see the same per-statement scope state as resolve. fnret is
threaded through resolvefnbody so ? can find the enclosing return.

New test/wcc/950_selfcheck.c — five rows exercising each error path
(missing variant, non-tagged enclosing, missing error subset
member, the flag-aware happy path, the flag-aware missing-error
case). Test suite now reports 21 ok.

Makefile

@@ -215,7 +215,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_at_test \
 	  $(BIN)/test_selfhost $(BIN)/test_w6a_ww $(BIN)/test_w6l_ww \
 	  $(BIN)/test_w6c_ww $(BIN)/test_ww_ww $(BIN)/test_self_rebuild \
-	  $(BIN)/test_dyn_ww
+	  $(BIN)/test_dyn_ww $(BIN)/test_selfcheck
 
 $(BIN)/test_smoke: test/wcc/000_smoke.c $(LIB)/libwcc.a | $(BIN)
 	$(CC) $(CFLAGS) $(INCS) -o $@ $< -L$(LIB) -lwcc
@@ -283,6 +283,9 @@ $(BIN)/test_self_rebuild: test/wcc/995_self_rebuild.c $(BIN)/ww_ww \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_selfcheck: test/wcc/950_selfcheck.c $(BIN)/wwdump_ww | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_dyn_ww: test/wcc/996_dyn_ww.c $(BIN)/ww $(BIN)/w6l $(BIN)/w6l_ww \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<