prep docs and service updates

docs/modules/policy/prep/2025-11-20-change-events-prep.md

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+# Change Events Prep — PREP-POLICY-ENGINE-30-003-CHANGE-EVENTS-DEPEN
+
+Status: Draft (2025-11-20)
+Owners: Policy Guild · Scheduler Guild · Cartographer Guild
+Scope: Define the change-event payload and scheduling expectations following simulation bridge (30-002).
+
+## Dependencies
+- Simulation bridge output schema (30-002).
+- Scheduler delivery channel (NATS/Redis) subject names and dedupe policy.
+
+## Draft event envelope
+- `event_type`: `policy.overlay.change`
+- Fields: `tenant_id`, `overlay_hash`, `policy_profile_hash`, `simulation_id?`, `changes[]` (array of `{component_purl, advisory_id, prev_status, new_status, severity_delta?, trace_ref}`), `occurred_at` (UTC), `event_id` (deterministic ID = hash of overlay_hash + timestamp).
+- Transport: propose NATS subject `policy.overlay.change` with durable stream; idempotency key = `event_id`.
+- Observability: counter `policy_overlay_change_total{tenant,result}`; log template includes overlay_hash, event_id, change_count.
+
+## Acceptance to close PREP
+- Subject/stream names confirmed with Scheduler.
+- JSON schema stub saved at `docs/modules/policy/schemas/policy-overlay-change@draft.json`.
+- Sample event at `docs/modules/policy/samples/policy-overlay-change@draft.json`.
+
+## Handoff
+This document serves as the prep artefact for PREP-POLICY-ENGINE-30-003-CHANGE-EVENTS-DEPEN. Update once 30-002 finalizes schema/subject; then unblock the implementation task.