cstage+test: store .len/.cap on every variadic-pack element (#16)

cstage variadic gather stored only AX (.ptr) per element; .len and
.cap read stack residue at the callee. Tagged-union variadic path
escaped because cg_widen_tagged_store wrote the full slot — but
primitive-type variadics (str..., slice...) silently dropped the
trailing fields. Selfhost only uses tagged-union variadics
(formattable...) so bootstrap byte-id ww2==ww3==ww4 stayed green;
the bug surfaced in worker-strings pre-flight (session 5) on the
Hare-faithful concat(strs: str...) shape.

Per-element store branch now mirrors selfhost/cmd/wcc/cgenexpr.ww
velemstr (AX→slot+0, BX→slot+8) and velemslice (AX→slot+0,
BX→slot+8, CX→slot+16). Also swap dname-before-sname allocation
order in the variadic-pack frame layout to match wwstage scanlocals
+ localadd order (cgendecl.ww:507-516 and cgenexpr.ww:2949-2954);
without the swap post-fix asm has correct stores at mismatched
offsets vs wwstage.

Rule-10 alignment: cstage UP to wwstage's already-correct primitive
variadic path.

743_variadic_pack pins the contract: asm-presence ≥3 ptr-stores +
≥3 len-stores in caller TEXT on both stages, plus cs-vs-ws cmp -s
byte-id per row. 117/117 ok. Bootstrap byte-id ww2==ww3==ww4 holds.

Unblocks: lib/bytes contains-variadic, lib/strings sub variadic,
and the concat/trim/contains family that c1 shipped non-variadic.

Makefile

@@ -275,6 +275,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_chained_write \
 	  $(BIN)/test_dotbase_chained \
 	  $(BIN)/test_parse_error \
+	  $(BIN)/test_variadic_pack \
 	  $(BIN)/test_fnparams_bare_leaf_shadow \
 	  $(BIN)/test_fnret_bare_leaf_shadow \
 	  $(BIN)/test_param_shadow_mod \
@@ -664,6 +665,10 @@ $(BIN)/test_parse_error: test/wcc/742_parse_error.c \
 		$(BIN)/w6c $(BIN)/w6c_ww | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_variadic_pack: test/wcc/743_variadic_pack.c \
+		$(BIN)/w6c $(BIN)/w6c_ww | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_fnparams_bare_leaf_shadow: test/wcc/732_fnparams_bare_leaf_shadow.c \
 		$(BIN)/w6c $(BIN)/w6c_ww | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<