Add Outline API client library foundation

Implements a first vertical slice of the Outline RPC API (auth, documents,
collections, users) covering the structural patterns used across the whole
API: single object, paginated list, tree, create/update, delete, search.
Chosen as the basis for future CLI and GUI clients built on top of this crate.

- Handwritten client (not codegen) against the vendored OpenAPI spec, since
  Outline's API is uniformly POST /api/<resource>.<action> with JSON bodies
  and inline/anonymous schemas that generators handle poorly
- Async (reqwest + tokio) Client, cheaply cloneable, no &mut self methods
- thiserror-based Error with ErrorKind classification and boxed API error
  context; forward-compatible models (unknown fields ignored, unknown
  string-enum values preserved via a catch-all variant) since the API is
  unversioned and self-hosted instances vary in age
- Pagination via Page<T>/Paginator with next_page/collect_all/into_stream
- wiremock-based test suite plus a spec-coverage test guarding against typos
  in RPC method names

tests/models.rs

@@ -0,0 +1,101 @@
+use outline::models::{Ability, DocumentTasks, NavigationNode, Policy, User, UserRole};
+use serde_json::json;
+
+#[test]
+fn deserializes_full_document_fixture() {
+    let raw = include_str!("fixtures/document.json");
+    let document: outline::models::Document = serde_json::from_str(raw).unwrap();
+
+    assert_eq!(document.title, "Welcome to Acme Inc");
+    assert_eq!(document.url_id.as_deref(), Some("hDYep1TPAM"));
+    assert_eq!(
+        document.tasks,
+        Some(DocumentTasks {
+            completed: 1,
+            total: 4
+        })
+    );
+    assert!(document.parent_document_id.is_none());
+    assert!(document.archived_at.is_none());
+    assert_eq!(document.created_by.unwrap().name, "Jane Doe");
+}
+
+/// A server (self-hosted, newer than this crate) adding a brand new field to
+/// a response must not break deserialization of the rest of the object.
+#[test]
+fn unknown_top_level_field_is_ignored() {
+    let raw = json!({
+        "id": "doc-1",
+        "title": "Still works",
+        "aBrandNewFieldFromTheFuture": { "nested": true },
+    });
+
+    let document: outline::models::Document = serde_json::from_value(raw).unwrap();
+    assert_eq!(document.title, "Still works");
+}
+
+/// A server returning a role value this crate doesn't know about yet must
+/// round-trip instead of failing to deserialize the whole `User`.
+#[test]
+fn unknown_enum_value_round_trips_via_unknown_variant() {
+    let raw = json!({
+        "id": "user-1",
+        "name": "Future User",
+        "role": "super_admin",
+    });
+
+    let user: User = serde_json::from_value(raw).unwrap();
+    match &user.role {
+        Some(UserRole::Unknown(value)) => assert_eq!(value.0, "super_admin"),
+        other => panic!("expected UserRole::Unknown, got {other:?}"),
+    }
+
+    let round_tripped = serde_json::to_value(&user).unwrap();
+    assert_eq!(round_tripped["role"], json!("super_admin"));
+}
+
+#[test]
+fn known_enum_value_deserializes_normally() {
+    let raw = json!({ "id": "user-1", "name": "Jane", "role": "member" });
+    let user: User = serde_json::from_value(raw).unwrap();
+    assert_eq!(user.role, Some(UserRole::Member));
+}
+
+#[test]
+fn policy_ability_accepts_boolean_or_membership_list() {
+    let raw = json!({
+        "id": "doc-1",
+        "abilities": {
+            "update": true,
+            "delete": false,
+            "restrictedUpdate": ["group-a", "group-b"],
+        }
+    });
+
+    let policy: Policy = serde_json::from_value(raw).unwrap();
+    assert!(policy.can("update"));
+    assert!(!policy.can("delete"));
+    assert!(policy.can("restrictedUpdate"));
+    assert!(!policy.can("missingAbility"));
+    assert_eq!(
+        policy.abilities["restrictedUpdate"],
+        Ability::Memberships(vec!["group-a".into(), "group-b".into()])
+    );
+}
+
+#[test]
+fn navigation_node_deserializes_recursively() {
+    let raw = json!({
+        "id": "doc-1",
+        "title": "Parent",
+        "children": [
+            { "id": "doc-2", "title": "Child", "children": [
+                { "id": "doc-3", "title": "Grandchild" }
+            ]}
+        ]
+    });
+
+    let node: NavigationNode = serde_json::from_value(raw).unwrap();
+    assert_eq!(node.children.len(), 1);
+    assert_eq!(node.children[0].children[0].title, "Grandchild");
+}