wcc-ww: assert/abort builtins — checker tag + cgen rt_abort lowering (#58)

wwstage had no EXPR_ASSERT-family intercept: the checker left bare
assert/abort calls untyped (asserttyped gate 4 skipped them by design)
and cgcall fell through to the regular call path, emitting
CALL assert(SB) for a symbol that exists nowhere — link-fail. cstage
was already correct (tag ty_err at check.c:1536-1572, lower inline via
rt_abort at cgen.c:6618-6663).

Mirror the same tag-then-lower pair: exprtype N_CALL stamps the call
void and the callee TY_ERR behind the scopelookupprefer no-shadow gate
(the isassertfam predicate), with the cstage arg diagnostics (cond must
be bool, msg must be str, arity caps); cgcall keys on the TY_ERR tag
and emits the identical CMPQ/JNE/rt_abort sequence. A user-shadowed
assert/abort (same-module or cross-module, the #45 shape, task #14)
stays untagged on the regular call path — byte-id for all existing lib
code preserved.

The cond check does NOT alias-peel: cstage compares ty_bool by
identity (check.c:1560), so `type myb = bool` is rejected there;
wwstage aligns down per rule 10 (a resolvealias here was accepting it
— cs/ww accept-reject divergence). Widening both stages together
belongs to the alias-peel choke-point arc (task #5, #47/#68).

The resolvewalk N_IDENT resolution counter learns the builtin shape:
an unshadowed abort/assert ident binds no sym BY DESIGN, so wwdump
-r's zero-unresolved gate (990 probe 4) counts it resolved instead of
failing builtin-using units.

test 957: 13 rows — pass/fail/msg/bare abort (run exit + rt_abort
stderr content; no-msg rows pin EMPTY stderr = the (NULL,0) shape),
assert in an imported module, same-module + cross-module shadow
controls, 5 checker rejects pinned on diagnostic CONTENT (shared
substring; cstage prefixes pos, wwstage cerr is bare) incl. the
alias-of-bool cond row pinning the rule-10 down-alignment; each
positive row pins cstage run exit + cs==ww byte-id. On pre-fix master
11/13 rows trip (survivors = the two shadow controls).

Residual (separate root, deferred diagnostic class): zero-arg assert()
is not intercepted by either stage; cstage rejects via the generic
undefined-ident path, wwstage's undefined-callee diagnostic is the
class deferred behind wiring checkfile into w6c_ww.

Makefile

@@ -459,6 +459,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_f32stamp_run \
 	  $(BIN)/test_f32arg_run \
 	  $(BIN)/test_tuprecv_f64_run \
+	  $(BIN)/test_assert_builtin_run \
 	  $(BIN)/test_modqualdestr_run \
 	  $(BIN)/test_tupparam_run \
 	  $(BIN)/test_structparam_run \
@@ -2023,6 +2024,11 @@ $(BIN)/test_tuprecv_f64_run: test/wcc/956_tuprecv_f64_run.c $(BIN)/ww \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_assert_builtin_run: test/wcc/957_assert_builtin_run.c $(BIN)/ww \
+		$(BIN)/w6c $(BIN)/w6c_ww $(BIN)/w6a $(BIN)/w6l \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_modqualdestr_run: test/wcc/956_modqualdestr_run.c $(BIN)/ww \
 		$(BIN)/w6c $(BIN)/w6c_ww $(BIN)/w6a $(BIN)/w6l \
 		$(LIB)/libwwrt.a | $(BIN)