[iterate-4A] jit: opcode-frequency histogram (XENIA_JIT_HIST) for coverage targeting

Gated 1-in-16 sampled opcode histogram in the recompiler (tallied in run_block,
printed sorted with cumulative % at clean exit). Drives which opcodes the JIT
lowers first. Boot+movie result: workload is FP-heavy — addi 15%, lwz 12%,
rlwinm 10%, lfs 9%, stfs 7.5%, lfsx 5%, fmaddsx 4.6%, bc 3.7%, fmulsx 3.6%,
stw 3.4% ... top-20 = 90%, ~33% floating-point. This is why we go straight to a
Cranelift machine-code JIT (which attacks the FP op bodies) rather than a
closure-threaded stage (which only removes dispatch).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/xenia-app/src/main.rs

@@ -4396,6 +4396,8 @@ fn dump_thread_diagnostic(
     }
     // JIT differential harness: checked/skipped/mismatch tally at clean exit.
     xenia_cpu::recompiler::report_diff_summary();
+    // JIT opcode-frequency histogram (XENIA_JIT_HIST) — drives M1 coverage.
+    xenia_cpu::recompiler::report_histogram();
 
     // STEP-10 diagnostic (observe-only, env-gated `XENIA_DUMP_SLOTS=1`).
     // Prints each scheduler slot's full runqueue with the fields needed to