SystemVerilog DPI capture: packed-vector groups don't cover unpacked arrays, reals or strings

A common design for fast simulation-time waveform capture is a generated SV observer that concatenates a group of packed signals and passes them to C through one fixed-width DPI import (svBitVecVal/svLogicVecVal), called when any member changes. This is efficient, but it silently limits what can be captured: - Unpacked arrays/memories: a packed concatenation can't take them directly; generate one tap per element (or a loop with an index argument) and give each element its own signal ID. - real/shortreal: use a separate DPI import with a real argument (or $realtobits into a 64-bit packed value) and mark the signal as real in the writer. - string: needs a string/chandle-style DPI argument and a variable-length value path. - class objects aren't plain signal values; decide explicitly what (if anything) to record. Keep the distinction clear in docs: these are limits of the capture observer, not necessarily of the waveform format, which may already store reals, strings and per-element arrays when converting from VCD/FSDB.