P135 - Cache background policy audit

P135 keeps P134’s guarded aux-load policy and adds mutually exclusive background-block attribution. This is a measurement rung, not a speed rung.

The shell workload passed:

P135-FILE-OK @ cycle 219,445,544
post-load cycles  : 219,445,401
shell window      : 65,411,939
instr retired     : 86,534,503
CPI               : 2.5359

The useful result is the bucket split:

aux-load candidates           : 5,418,028
frontend not safe/useful      : 1,596,970
frontend-ready candidates     : 3,821,058
background quiet / issued     : 140,540
D-cache-only background block : 196,378
I-cache-only background block : 1,620,547
both backgrounds block        : 1,863,593
queue full drops              : 0
aux errors/cancels            : 0 / 0

So P136 should not start by relaxing the D-cache guard. The larger single limiter is I-cache background fill.