Add periodic KV-canary stats logging and kernel-run-counter health check

[fzyzcjy]

Add two always-on KV-canary observability add-ons, both pure observers
of CanaryDeviceState counters and the sweep orchestrator (nothing
depends on their output):

• Periodic stats logging
  (python/sglang/srt/kv_canary/runner/stats_logger.py): a
  PeriodicCanaryStatsLogger that prints, every N forward steps, the
  number of protected tokens, sweep passes, cumulative violations and
  the count of active launch tags, driven once per outer step via a
  delayed D2H handler so the read never blocks the forward. Gated by
  CanaryConfig.stats_print_every_n_steps (env
  SGLANG_KV_CANARY_STATS_PRINT_EVERY_N_STEPS, 0 disables).

• Kernel-run-counter health check
  (python/sglang/srt/kv_canary/runner/health_checker.py): a
  KernelRunCounterHealthChecker that watches the per-tag kernel run
  counters and warns when a canary kernel stops advancing (e.g. a tag
  silently goes un-launched), catching wiring regressions that would
  otherwise pass silently. Unit test in
  test/registered/kv_canary/test_self_unit_runner_health.py.

Both are constructed alongside the other per-step runners in
CanaryManager and ticked once per outer step.

---

CI States

Latest PR Test (Base): 🚫 Run #26700542488
Latest PR Test (Extra): ❌ Run #26700542417

[fzyzcjy]

/tag-and-rerun-ci

[gemini-code-assist]

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[fzyzcjy]

✅ verify-rebased: PASS

Rebased source tree matches the PR head tree (chain content lives on the PR branch; main does not yet reflect the last group).

	SHA	Tree
Rebased source (tag verify/rebased/20260531T015946Z)	34bc9ecc86ed	c643a57b595e822a34161d550bb4043d0c06addb
PR head (tom/pr_chain/tom/kv_canary_revert_reversed/add-periodic-kv-canary-stats-logging-and-kernel-run-counter-health-check)	24a51e3faf13	c643a57b595e822a34161d550bb4043d0c06addb
upstream/main	7dd19ae3d8ba	35c87847a6bdf8cec6fa63a2eeea9740061a68d6

Reproduce locally (the rebase tag persists after this run):

git fetch upstream 24a51e3faf13dd35365e39542677cb4496c93dcd
REB_TREE=$(git rev-parse 'verify/rebased/20260531T015946Z^{tree}')
PR_TREE=$(git rev-parse '24a51e3faf13dd35365e39542677cb4496c93dcd^{tree}')
MAIN_TREE=$(git rev-parse 'upstream/main^{tree}')
echo "REB_TREE  = $REB_TREE"
echo "PR_TREE   = $PR_TREE"
echo "MAIN_TREE = $MAIN_TREE"

Generated by single_commit_pr_chain.py verify-rebased.