test(framework): Add k8s demo cardinality proof

[msheller]

Summary

• Adds F7e demo-friendly cardinality proof for the local k8s harness.
• Exposes active Pod budget as a wrapper parameter for capacity proof runs.
• Adds a --demo preset for active Pod budget 2, three seeded ServerApp runs, a 30-second probe hold, and skipped namespace cleanup for inspection.
• Adds probe ServerApp start/sleep/exit log lines with context.run_id while preserving the existing verifier marker substring.
• Extends evidence and verifier checks for the budget-2/three-task proof: two TaskExecutors can be active, the third waits, one slot opens, and the third launches.

Stack

• Stacked on test(framework): Add k8s capacity cleanup proof #7406.
• Base branch: codex/f7-integrated-k3d-capacity-cleanup-proof-v2.

Evidence

Real local k3d demo run passed and wrote evidence to:

/private/tmp/f7e-demo-cardinality-proof-20260617-codex-escalated

Verifier rerun:

python framework/dev/k8s/verify_evidence.py \
  /private/tmp/f7e-demo-cardinality-proof-20260617-codex-escalated \
  --expected-result local-k8s-capacity-cleanup-proof \
  --expected-active-pod-budget 2 \
  --expected-seed-run-count 3 \
  --no-require-cleanup

Result: Verification: PASSED.

The demo preset leaves namespace resources in place by design. After evidence capture and review, the local flower-local-k8s namespace was deleted.

Validation

Implementation thread reported:

bash -n framework/dev/k8s/test-real-launch-path.sh
python -m py_compile <touched Python files>
python -m ruff check <touched Python files>
python -m isort --check-only <touched Python files>
python -m pytest framework/py/flwr/supercore/superexec/executor/kubernetes_executor_harness_test.py
git diff --check
framework/dev/k8s/test-real-launch-path.sh --demo --skip-build --output-dir /private/tmp/f7e-demo-cardinality-proof-20260617-codex-escalated

Lead thread additionally reran:

python framework/dev/k8s/verify_evidence.py \
  /private/tmp/f7e-demo-cardinality-proof-20260617-codex-escalated \
  --expected-result local-k8s-capacity-cleanup-proof \
  --expected-active-pod-budget 2 \
  --expected-seed-run-count 3 \
  --no-require-cleanup
git diff --check