P122 - Multi-entry ROB sketch

P122 took the one-entry ROB lifetime record from P121 and made it a four-entry ring. This is still shadow-only; architectural writes still go through the in-order regs[] path.

The BusyBox shell workload passed:

P122-FILE-OK @ cycle 219,016,930
post-load cycles  : 219,016,787
shell window      : 65,109,744
instr retired     : 86,435,662
CPI               : 2.5339

The useful result is occupancy:

ROB capacity       : 4
max occupancy      : 1
allocs             : 59,300,128
commits            : 59,299,949
flushes            : 179
full waits         : 0
missing commits    : 0
occupancy 0 cycles : 135,022,896
occupancy 1 cycles : 83,993,890
occupancy 2 cycles : 0
occupancy 3 cycles : 0
occupancy 4 cycles : 0

That is the right negative result. The core cannot fill a larger ROB while decode, execute, and writeback are still serialized. The next backend experiment should split dispatch/issue from commit before making the ROB bigger or pretending a scheduler exists.