cbtop pcie collector counts 63 (unknown width sentinel) as max_width; test_pcie_collector fails on kfocus-mq, ejecting MG runs

MG run 40112818334 shard log: test_pcie_collector asserts max_width <= 16. kfocus reports current_link_width 63 on 4 devices (measured by ssh: 0x3, 1x4, 4x11, 8x3, 63x4). Not a parse bug: 63 is an unknown-width sentinel. Fix: count only legal PCIe widths {0,1,2,4,8,12,16,32}; inject the sysfs root and test with fixtures.