docs(httputil): correct tier from 3 to 2

httputil depends on xerrors (Tier 0) and valid (Tier 1), placing it at
Tier 2. No infrastructure or lifecycle dependencies exist in this module.

README.md

@@ -0,0 +1,59 @@
+# httputil
+
+Typed HTTP handler adapters and response helpers for stdlib `net/http`.
+
+## Install
+
+```
+go get code.nochebuena.dev/go/httputil
+```
+
+## Typed handlers
+
+```go
+// JSON body + validation + typed response
+r.Post("/orders", httputil.Handle(validator, svc.CreateOrder))
+
+// No request body (GET / DELETE)
+r.Get("/orders/{id}", httputil.HandleNoBody(svc.GetOrder))
+
+// Request body, no response body (204)
+r.Delete("/orders/{id}", httputil.HandleEmpty(validator, svc.DeleteOrder))
+
+// Manual handler with centralised error mapping
+r.Get("/raw", httputil.HandlerFunc(func(w http.ResponseWriter, r *http.Request) error {
+    data, err := svc.Load(r.Context(), chi.URLParam(r, "id"))
+    if err != nil {
+        return err
+    }
+    return httputil.JSON(w, http.StatusOK, data)
+}))
+```
+
+## Error mapping
+
+`xerrors.Code` → HTTP status:
+
+| Code | Status |
+|---|---|
+| `ErrInvalidInput` | 400 |
+| `ErrUnauthorized` | 401 |
+| `ErrPermissionDenied` | 403 |
+| `ErrNotFound` | 404 |
+| `ErrAlreadyExists` | 409 |
+| `ErrInternal` | 500 |
+| `ErrNotImplemented` | 501 |
+| `ErrUnavailable` | 503 |
+| `ErrDeadlineExceeded` | 504 |
+| unknown | 500 |
+
+Error response body:
+```json
+{"code": "NOT_FOUND", "message": "record not found"}
+```
+Fields from `xerrors.Err` are merged into the top-level response object.
+
+## Dependencies
+
+- `code.nochebuena.dev/go/xerrors`
+- `code.nochebuena.dev/go/valid`