[MoonEP] Log host capacity utilization

[musclemuller]

Summary

• add per-dispatch DEBUG key=value logs for phase, rank, layer, token capacity, padding, utilization, and static padding ratio
• thread MoeRunnerConfig.layer_id only into the MoonEP inner dispatcher and reject missing layer identity
• preserve the existing over-capacity ValueError without emitting a normal capacity record

Scope

This is the independent host-side capacity-observability slice from #35783. It is based directly on the moonep branch and does not depend on #35834 or #34874.

Duplicated-expert statistics, prefetch volume, metrics aggregation, symmetric-pool lifecycle, and GPU/protocol work remain follow-ups.

Testing

• 13 focused CPU tests in test_moonep_buffer.py
• Ruff 0.15.1: F401, F821, UP037
• isort 7.0.0
• Black 26.1.0
• registered-test registry check

Part of #35783.

---

CI States

Latest PR Test (Base): ❌ Run #32511192256
Latest PR Test (Extra): ❌ Run #32511192175
Latest PR Test (AMD ROCm 10): ❌ Run #32511192671

[musclemuller]

CI is currently blocked before tests by the repository-wide MIN_BASE_SHA gate, which requires main@aa3f76679930. This PR intentionally targets the moonep staging branch at 1334810, as requested, so the branches are expected to diverge. Would you prefer to apply the bypass-maintenance label to this PR, or update the moonep branch? I have not rebased onto main because that would change the intended base and scope.