Fix O(N) idle-loop messaging pump; full 3x9x3 turn-attractor campaign clean on all 3 architectures (FABRIC-3.md §XXII)

sk_repl_idle()'s messaging pump (repl.c) walked the entire live-VM
registry every idle beat (~1Hz) and dispatched a full VM-EXEC
"MSG-TICK" -- a dictionary lookup plus a 32-slot arena scan -- into
every live VM, every tick, unconditionally, forever. Fine at Tripod's
original 3-VM scale; a full 9-identity turn-attractor campaign
exposed it as a genuine wall on riscv64 specifically (its TCG makes
each dispatch cost more): the same campaign that completed in 194s/
339s on amd64/aarch64 never finished on riscv64 at 9 VMs across three
attempts, while 8 VMs there was fine in 159s.

Ruled out capacity explanations before touching anything: bumping
riscv64's QEMU RAM 1024->4096 changed nothing (reverted), and a live
STADIUM-RES@/MSG-STATUS probe with all 9 VMs attached showed no
depletion. Host memory pressure was also ruled out directly (one
background task did get OOM-killed once during the investigation,
but the identical stall reproduced again with 9.2GB free). The real
mistake was three premature kills under 4 minutes with no way to
tell "slow" from "stuck" from outside the guest -- fixed by having
run_doe_batch.sh sample the qemu process's own /proc/<pid>/stat utime
every 60s; with that signal, riscv64 at 9 VMs was unambiguously alive
(climbing utime, no hang), just disproportionately slow going from 8
VMs (159s) to 9 (600s+ and climbing).

This was never really a riscv64-only bug: an O(N) per-second walk
over the full VM population doesn't scale to the hundreds of VMs this
fleet is headed toward, on any architecture -- riscv64 just made it
visible first, at N=9, because its per-dispatch cost is highest.

Fixed by round-robin batching: the pump now dispatches to at most
SK_MSG_PUMP_BATCH (4) live VMs per idle beat via a persistent cursor
that resumes where the previous beat left off, instead of all of them
every time. Bounds both the scan and dispatch cost to O(K) regardless
of total VM count; any single VM's queue now drains roughly every
ceil(N/K) beats instead of every beat, still bounded and still
matching the pump's own existing best-effort contract. No new C
primitives, no messaging/Stadium changes.

Verified: clean build on all 3 architectures, then the full 3x9x3
campaign re-run on all 3 (not just riscv64) per the standing rule
that a defect repair requires a clean re-run everywhere before
anything counts as closed:

  amd64   198s  0 faults  99/99 tokens x8  656/656 K-conserved
  aarch64 339s  0 faults  99/99 tokens x8  659/659 K-conserved
  riscv64 178s  0 faults  99/99 tokens x8  659/659 K-conserved

riscv64 went from "never completes" to faster than aarch64, same
campaign, same seed, same identity set. No regression on amd64/
aarch64.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EXieurDfDSsDFdnSyusuWo

experiments/bare_metal/runs/doe-riscv64-20260912-174936.csv

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