Bring in the completed stack cache feature (from .vhd.new) that was
started but not committed. Adds MAX_STCACHE_BITS generic, stack cache
signals, CACHEST instantiation, and associated logic. Fixes EVO and
EVO_MINIMAL FPGA build failures caused by zpu_soc.vhd referencing
the missing MAX_STCACHE_BITS parameter.
Previous version saved as zpu_core_evo.vhd.pre_stcache.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>