Add shared-root destination state model

docs/config.md

@@ -255,7 +255,7 @@ Source bundle digest mismatches fail validation before destination writes occur.
 
 ## Destination Fields
 
-Each destination embeds a backend config at the destination level and may also configure publishing, transforms, path mapping, links, reconciliation, and transfer behavior.
+Each destination embeds a backend config at the destination level and may also configure publishing, transforms, path mapping, links, state, reconciliation, and transfer behavior.
 
 ```yaml
 destinations:
@@ -267,6 +267,8 @@ destinations:
       html: false
     path_mapping:
       mode: preserve_relative
+    state:
+      mode: single_owner
     reconciliation:
       mode: replace
     transfer:
@@ -282,6 +284,7 @@ destinations:
 - `transform`: required only when publishing generated HTML.
 - `path_mapping`: optional destination path mapping policy.
 - `links`: optional public URL metadata policy.
+- `state`: optional destination state ownership policy.
 - `reconciliation`: optional managed-output reconciliation policy.
 - `transfer`: optional destination comparison action policy.
 
@@ -359,6 +362,19 @@ Primary URL policies:
 
 If no output matches the primary policy, per-output URLs may still be recorded and the top-level primary URL is omitted.
 
+## Destination State Policy
+
+```yaml
+state:
+  mode: single_owner
+```
+
+- `state.mode`: optional. Accepted values are `single_owner` and `shared_root`; default is `single_owner`.
+
+`single_owner` state records one pipeline/destination owner for each destination bundle path and is the state mode written by `run`.
+
+`shared_root` is accepted by configuration validation and by destination state parsing for shared-root `.distributor.json` files. Current publish execution writes single-owner destination state.
+
 ## Reconciliation Policy
 
 ```yaml
@@ -448,6 +464,7 @@ Defaults are applied after YAML decoding and before validation:
 - `transform.markdown_to_html.mode: sidecar` when a Markdown transform block is present and mode is omitted
 - `path_mapping.mode: preserve_relative`
 - `links.primary: auto` when a `links` block is present and `primary` is omitted
+- `state.mode: single_owner`
 - `reconciliation.mode: replace`
 - `transfer.on_destination_same: skip`
 - `transfer.on_destination_older: replace`