P125 - Source-ready scoreboard model

P125 adds a shadow architectural register busy-bit scoreboard. It marks destinations on shadow ROB allocation, clears them at shadow commit, and clears everything on trap flush. Architectural execution is unchanged.

The BusyBox shell workload passed:

P125-FILE-OK @ cycle 218,418,928
post-load cycles  : 218,418,785
shell window      : 64,495,264
instr retired     : 86,247,293
CPI               : 2.5325

The scoreboard found:

candidates        : 24,405,973
all sources ready : 13,098,705
rs1 busy          : 7,756,459
rs2 busy          : 3,550,809
both busy         : 0
dest busy         : 6,358,845
max busy regs     : 1
marks/clears      : 59,172,706 / 59,172,527
flush clears      : 179

The result keeps us honest: source readiness is now visible, but the machine is still one-deep. The next useful backend model is separate memory/control holding records, not a larger integer queue.