feat: define policy impact subject contract

docs/POLICY_CONTRACTS.md

@@ -126,6 +126,27 @@ When the capability is absent, modules must not silently emulate cross-scope
 inheritance. Their conservative fallback is limited to local tenant
 definitions and disables reuse, derivation, and automation.
 
+## Bounded Impact-Subject Providers
+
+Policy impact previews discover optional subject providers through capability
+names beginning with `policy.impactSubjects.`. The suffix is the stable
+provider ID; for example, Views contributes `policy.impactSubjects.views`.
+Providers implement `PolicyImpactSubjectProvider` and receive a
+`PolicyImpactPopulationRequest` containing the active tenant, policy family,
+an explicit selector, actor scopes, detail-disclosure decision, and a limit of
+at most 500. They return `PolicyImpactSubjectBatch` with unique opaque subject
+references and an explicit `complete`, `sampled`, `truncated`, or `unavailable`
+state. An unavailable batch must explain the gap, and a total may never be
+smaller than the returned subject count.
+
+Core does not scan module data or evaluate domain policy. The owning module
+selects and permission-filters its candidates; Policy compares the current and
+proposed decisions and controls response disclosure. A caller must select one
+or more provider populations explicitly. This preserves optional-module
+boundaries and prevents a seemingly harmless preview from becoming an
+unbounded platform query. Providers must not include credentials, secrets, or
+unfiltered cross-tenant labels in subject attributes.
+
 ## Frontend Contract
 
 Policy UIs must:
@@ -138,6 +159,9 @@ Policy UIs must:
   lower-level limit to `false`
 - avoid sending locked fields or re-enable attempts in save payloads
 - show inherited values separately from local overrides
+- require a current impact preview before enabling a governed high-impact save,
+  preserve its proposal hash on commit, and explain incomplete population
+  coverage rather than presenting unavailable providers as zero impact
 
 The core WebUI helper `privacyRetentionParentAllowsField()` centralizes the
 field-lock decision used by the retention editor and its lightweight module