iter4 fix: B-slice L1 reflist .fields copy-paste bug

In h264_va_slice_to_v4l2, the B-slice L1 reflist loop wrote .fields
into ref_pic_list0[i] instead of ref_pic_list1[i]. This corrupted L0
reflist fields when L1 was being built and left ref_pic_list1[i].fields
zero (which the kernel may interpret as "no valid field reference").

Pre-existing pre-iter4 bug (caught by iter4 Phase 5 sonnet review,
finding C2). Latent on hantro bbb_1080p30 in FRAME_BASED mode because
hantro walks reference_ts directly and ignores SLICE_PARAMS.fields,
but the bug is wrong-by-construction and would surface on any driver
that reads SLICE_PARAMS reflist fields, on interlaced content, or in
SLICE_BASED decode mode.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/h264.c

@@ -700,7 +700,7 @@ static void h264_va_slice_to_v4l2(struct request_data *driver_data,
 				continue;
 
 			slice->ref_pic_list1[i].index = idx;
-			slice->ref_pic_list0[i].fields = fields;
+			slice->ref_pic_list1[i].fields = fields;
 		}
 	}