Naming, layout and comments: descriptive full-word names, one item per line in wrapped calls and literals, and doc comments on exported names

Short and single-letter names in application code become full words (tests
keep theirs). Wrapped calls, signatures and composite literals put one item
per line, and one-line function bodies are expanded. Exported names and
groups of constants get doc comments, and comments refer to the renamed
identifiers.

internal/httpx/problem.go

@@ -18,17 +18,17 @@ type Problem struct {
 
 // WriteProblem writes a problem+json error response.
 func WriteProblem(w http.ResponseWriter, r *http.Request, status int, detail string) {
-	p := Problem{
+	problem := Problem{
 		Type:   "about:blank",
 		Title:  http.StatusText(status),
 		Status: status,
 		Detail: detail,
 	}
 	if r != nil {
-		p.RequestID = RequestID(r.Context())
+		problem.RequestID = RequestID(r.Context())
 	}
 	w.Header().Set("Content-Type", "application/problem+json")
 	w.Header().Set("X-Content-Type-Options", "nosniff")
 	w.WriteHeader(status)
-	json.NewEncoder(w).Encode(p) //nolint:errcheck,gosec // the client may have gone; nothing to do
+	json.NewEncoder(w).Encode(problem) //nolint:errcheck,gosec // the client may have gone
 }