cgen: str tuple-positional element read -> 3-word -- Phase 2 C4.6 S3 (both stages)

Reading a str-typed tuple element by position (t.N) loaded 2 words (ptr,len), dropping cap -- the C4.6 coda with NO adjacent slice-element arm to mirror. Author the 3-word triple directly to the canonical {ptr,len,cap} ABI (AX,BX,CX off the BP frame slot; add cap->CX at +16). Kind-gated (TY_STR / isstrtype, never size==24). cstage==wwstage byte-identical -- cross-stage identity is the correctness oracle here, since there is no local slice sibling.

test/wcc/935: table-driven runtime .cap-survives over let s = t.1; poison rides the return ABI (R8) into the slot's cap word; a CX-clobbering call ensures a 2-word read cannot coincidentally pass. fail-before/pass-after verified INDEPENDENTLY on both stages.

main.combined.ww regenerated via the canonical make path (md5-stable).

Makefile

@@ -257,6 +257,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_str_elem_cap_run \
 	  $(BIN)/test_str_field_cap_run \
 	  $(BIN)/test_str_chained_field_cap_run \
+	  $(BIN)/test_str_tuple_elem_cap_run \
 	  $(BIN)/test_composite_call_arg \
 	  $(BIN)/test_composite_call_arg_run \
 	  $(BIN)/test_letdecl_zeroinit \
@@ -641,6 +642,12 @@ $(BIN)/test_str_chained_field_cap_run: test/wcc/934_str_chained_field_cap_run.c
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_str_tuple_elem_cap_run: test/wcc/935_str_tuple_elem_cap_run.c \
+		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_composite_call_arg: test/wcc/723_composite_call_arg.c \
 		$(BIN)/w6c $(BIN)/w6c_ww | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<