Integrate Distribution Lists with Campaign recipients

docs/CAMPAIGN_HANDBOOK.md

@@ -412,6 +412,7 @@ Current principal contracts include:
 | `files.campaign_attachments` 0.1.x | Files -> Campaign | Select/materialize governed file versions and preserve campaign usage/evidence |
 | `addresses.lookup` 0.1.x | Addresses -> Campaign | Optional address suggestions |
 | `addresses.recipient_source` 0.1.x | Addresses -> Campaign | Optional versioned recipient-source snapshots |
+| `dist_lists.source` / `dist_lists.expand` 0.1.x | Distribution Lists -> Campaign | Discover, preview, and freeze reusable audiences without importing module internals |
 | `campaigns.access` 0.1.x | Campaign -> platform | Explain campaign access/existence without exporting ORM objects |
 | `campaigns.mail_policy_context` 0.1.x | Campaign -> Mail | Resolve campaign tenant/owner context for Mail policy |
 | `campaigns.delivery_tasks` 0.1.x | Campaign -> workers | Execute narrow queued send/append tasks |
@@ -421,6 +422,28 @@ Breaking payload or ownership changes require an interface-version bump and a
 release-composition alignment gate. Optional absence must be tested physically,
 not only hidden in navigation.
 
+### Reusable Distribution Lists
+
+When Distribution Lists is available, Recipient data offers a separate import
+dialog. The author selects a visible list revision, supplies declared
+parameters, requests candidate channels, and previews included, excluded,
+stale, ambiguous, suppressed, policy-blocked, and provider-unavailable results.
+The final action freezes an idempotent Distribution Lists snapshot and copies
+the resulting rows into the editable Campaign version.
+
+Each copied row retains the list and revision IDs, definition and expansion
+hashes, snapshot ID, source entry IDs, provider references, channel candidates,
+the one selected route where it is unambiguous, fallback candidates, and the
+decision explanation. Campaign-only fields, attachment rules, review state,
+and outcomes remain local to Campaign and never mutate the reusable list.
+
+A later list revision only raises a drift warning. Refresh is deliberate and
+uses append or replace; saving that changed Campaign version clears prior
+validation, build, review, and execution state through the normal content
+invalidation path. Postal-only or otherwise unsupported routes remain present
+in the frozen evidence but inactive until a compatible Campaign output path is
+configured.
+
 ### External API expectations
 
 - Tenant and campaign access are evaluated for every operation.