add empirically verified event, request, and metrics specifications

[ashum9]

Overview

Days 1–2 Contract Hard Gate specifications, empirically verified 1-to-1 against mofa-kernel/src/types.rs (origin/engine commit 5287b43).

Included Specs

• engine_event_schema.json: JSON Schema for all 10 EngineEvent SSE variants.
• inference_request_schema.json: Schema for InferenceRequest (locality, fallback_policy, reasoning).
• metrics_spec.md: 18-metric family taxonomy with mandatory provider and locality labels.

End-to-End Status

• Verified against Rust Kernel, Axum Engine Server, SSE streams, and React Frontend client.
• 100% compatible with mofa-observability collector & Prometheus metrics bridge.

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