cap trace value memcpy at scalar value size

[Nashit-h]

get_value_as in HalideTraceViz.cpp and Packet::get_value_as in HalideTraceUtils.h copy type.bits / 8 bytes of a trace value into a fixed 8-byte halide_scalar_value_t, but type.bits comes straight from the packet header in the trace stream. A crafted load/store packet declaring a scalar type wider than 64 bits makes the memcpy write up to 31 bytes into the 8-byte local, overflowing the stack before value_as ever checks the type. Cap the copy at sizeof(halide_scalar_value_t) at both sites; valid types (bits <= 64) copy exactly as before.

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)