Algorithmic-complexity DoS in EBR recursion (tetranacci explosion)

Summary

mbr_extended() bounds recursion DEPTH(>=16) but not BREADTH. Each EBR can spawn 4 EXT links. Self-referencing EBR (dp_start=0 all 4 entries EXT) forces ~11496 synchronous sector reads during disk attach (tetranacci T(1..15)). Slice-overflow guard(:490) prevents OOB write but does NOT prevent further EXT recording+recursion. Physical USB ~2min hang.