VPI value-change capture: writes just before $finish may never fire a callback; merge an end-of-sim snapshot

When building a waveform or capture tool on VPI cbValueChange callbacks, do not assume the callback stream is complete at the end of the run. A write in the same time step as $finish (or just before it) can end the simulation before the value-change callback is delivered, while a vpi_get_value read at end of sim (cbEndOfSimulation, or a final $finish-adjacent hook) still shows the new value. Fix: take an initial snapshot (cbStartOfSimulation, or right after registration) and a final snapshot, both with vpi_get_value, and merge them with the callback stream using last-value-wins at each time step. Test it with a small authored fixture that writes a few signals (1-bit, 33-bit and 65-bit) in the final step and then calls $finish. Also copy VPI-owned aval/bval buffers right away in the callback, and release lookup handles you create. Callback-only capture can pass all mid-run tests and still be red on this case.