[R9.3] Dependency-level explanations for posture diffs and serve mode

[a-a-k]

Context

Epic: R9. UX and explainability (#39)

Scope

Add dependency-level explanations for posture regressions and gate outcomes in batch and serve mode.

Current reports already expose aggregates, per-endpoint deltas, provenance, previous/baseline diffs, and topology/digest changes.
What is still missing is an explicit explanation path that ties a regression back to the resolved predicate/journey/dependency set that determined the outcome.

Definition of Done

• For a profile/endpoint result, Sheaft can surface the resolved predicate or journey/path semantics that determined success or failure.
• current-report / current-diff and at least one CLI or docs workflow show dependency-level explanations for posture changes, not just aggregate deltas.
• Tests and examples cover at least one snapshot -> diff -> explanation flow.