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/spec_coverage.rs

@@ -0,0 +1,38 @@
+//! Verifies that every RPC method name used by this crate still exists as a
+//! path in the vendored OpenAPI spec, catching typos and upstream renames.
+
+const IMPLEMENTED_METHODS: &[&str] = &[
+    "auth.info",
+    "auth.config",
+    "documents.info",
+    "documents.list",
+    "documents.search",
+    "documents.create",
+    "documents.update",
+    "documents.delete",
+    "collections.list",
+    "collections.info",
+    "collections.documents",
+    "users.list",
+    "users.info",
+];
+
+#[test]
+fn implemented_methods_exist_in_vendored_spec() {
+    let spec_raw = include_str!("../spec/spec3.json");
+    let spec: serde_json::Value = serde_json::from_str(spec_raw).unwrap();
+    let paths = spec["paths"]
+        .as_object()
+        .expect("spec should have a `paths` object");
+
+    let missing: Vec<&str> = IMPLEMENTED_METHODS
+        .iter()
+        .copied()
+        .filter(|method| !paths.contains_key(&format!("/{method}")))
+        .collect();
+
+    assert!(
+        missing.is_empty(),
+        "methods missing from vendored spec: {missing:?}"
+    );
+}