debug: add guard diagnostics in reactor entry path
- diag:pre-boot-reactor-with prints epoch summary before BOOT_REACTOR.with to pinpoint guard creation
- diag:step-hart-loop-inside tracks guard state inside step_hart_loop_at
- CSPRNG: fill_bytes auto-inits on first call (fixes exec_script tests panicking with CSPRNG not initialised)
- step_fork: fix test call arity (3 args after ipc merge)
- writev: yield guard between iovec iterations