wiki: add Routes Reference page

Routes-Reference.md

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+# Routes Reference
+
+`service.router()` registers exactly these routes:
+
+| Method | Path | Description |
+|--------|------|-------------|
+| `POST` | `/inbox` | Shared inbox — HTTP sig verification + dispatch, 1 MB limit |
+| `POST` | `/users/{id}/inbox` | Per-user inbox — same |
+| `GET` | `/users/{id}/outbox` | Cursor-based `OrderedCollection` |
+| `GET` | `/users/{id}/featured` | Pinned posts `OrderedCollection` |
+| `GET` | `/.well-known/webfinger` | JRD with `aliases` field |
+| `GET` | `/.well-known/nodeinfo` | Redirect to `/nodeinfo/2.0` |
+| `GET` | `/nodeinfo/2.0` | NodeInfo 2.0 document |
+
+---
+
+## Routes NOT registered
+
+`GET /users/{id}`, `GET /users/{id}/followers`, and `GET /users/{id}/following` are **not** registered by `service.router()`.
+
+These paths must serve both AP JSON (for federation) and your UI JSON (for your frontend) based on the `Accept` header. k-ap can't do the UI half, so your application owns the route and calls k-ap's helper methods for the AP half.
+
+---
+
+## Content negotiation pattern
+
+```rust
+use axum::{extract::{Path, State}, http::HeaderMap, response::IntoResponse, Json};
+
+async fn actor_handler(
+    Path(username): Path<String>,
+    headers: HeaderMap,
+    State(state): State<AppState>,
+) -> impl IntoResponse {
+    let user = state.db.find_user_by_username(&username).await?;
+
+    if wants_ap_json(&headers) {
+        // actor_json takes a UUID as a string
+        let json = state.service.actor_json(&user.id.to_string()).await?;
+        return (
+            [(axum::http::header::CONTENT_TYPE, "application/activity+json")],
+            json,
+        ).into_response();
+    }
+
+    // Return your UI JSON or HTML
+    Json(MyUserResponse::from(user)).into_response()
+}
+
+fn wants_ap_json(headers: &HeaderMap) -> bool {
+    headers
+        .get("accept")
+        .and_then(|v| v.to_str().ok())
+        .map(|v| v.contains("application/activity+json") || v.contains("application/ld+json"))
+        .unwrap_or(false)
+}
+```
+
+The same pattern for followers and following. `page` is `Option<u32>` — pass `None` for the collection root, `Some(n)` for a page:
+
+```rust
+// In your followers handler:
+let json = state.service.followers_collection_json(user.id, page).await?;
+
+// In your following handler:
+let json = state.service.following_collection_json(user.id, page).await?;
+```
+
+Both return JSON strings with the correct `@context` and `OrderedCollection` structure.