VCS DirectC vs DPI for C-side capture: vec32 field order is swapped, and DirectC has no value-change callbacks

From reading the VCS C-interface documentation while evaluating alternatives to SV DPI for passing many signal values to C (not yet measured): - DirectC (+vc, extern "C", headers generated with +allhdrs into vc_hdrs.h, plus the installed DirectC.h) can pass vector regs by reference, so the caller doesn't have to concatenate members into one wide argument. - ABI trap: in DirectC's vec32 the fields are c = control (bval) and d = data (aval), the opposite order from DPI's svLogicVecVal aval/bval. Never reinterpret one as the other; convert explicitly. Memory elements use yet another layout. - The documentation says DirectC cannot register value-change callbacks, so you still need generated HDL (an observer) that decides when to call. - Borrowed pointers are only promised valid during the call: compare and copy inside the call into your own buffer before any async queue. - Cheapest first test: one call with 65-bit values covering 0/1/X/Z, ascending and negative ranges, a net or port, a memory word and a time above 2^32. Check against a DPI reference before any timing. It needs a customer rebuild of their simv; it's not a runtime attach to an existing executable.