Add sourceosctl explain and evidence topology views

[mdheller]

Purpose

Implement the operator-facing explanation surface for SourceOS Mutation and Evidence Accountability, anchored to SourceOS spec PR #96:

SourceOS-Linux/sourceos-spec#96

Required commands

• sourceosctl explain writes
• sourceosctl explain sync
• sourceosctl explain browser
• sourceosctl explain terminal
• sourceosctl explain logs
• sourceosctl explain compromise

Required UI concepts

• Evidence Topology panel: event sources, sinks, routing, privilege requirements, redaction, sampling, drops, retention.
• Mutation Graph view: human intent, actor chain, object/path class, operation, policy, resource cost, causal parents, evidence quality, and downstream effects.

Acceptance criteria

• Operator can distinguish no positive compromise evidence from evidence sufficient to clear compromise.
• Resource incidents show delegated actors and missing evidence fields.
• UI/CLI can render at least one validated fixture from PR #96.