[P0] Emit signal quality and coverage report for every artifact

[a-a-k]

Context

Bering can emit valid model/snapshot artifacts from trace-only input, but generic traces do not infer retry policies, timeout policies, circuit-breaker limits, placement groups, or shared-resource references. Downstream tools such as Sheaft need an explicit quality signal before these artifacts are used as gate inputs.

Scope

Add a machine-readable signal-quality summary to every emitted model/snapshot or companion output.

The summary should report:

• which advanced fields are present, missing, inferred, or externally supplied;
• coverage for services, edges, endpoints, placements, resilience policy, shared resources, and observed timings;
• low-confidence trace-only cases that should not be treated as high-trust gate input.

Definition of Done

• Batch and runtime publication expose signal-quality data.
• Existing valid artifact contracts remain backward compatible.
• Docs explain how Sheaft and other downstream consumers should interpret the quality signal.
• Tests cover trace-only, topology_api, and overlay-enriched artifacts.