Default input mapping for binder routes.
Merges parsed query, body, and path objects into one input object. Path
keys win all collisions, then body keys, then query keys. Headers are never
merged: parsed headers include every raw request header, so merging them
would poison the use case input. The route binder passes a sole declared
input schema through unchanged when no other object input contains values;
this merge handles every other default mapping. Routes that combine a
non-object body with another source declare an explicit input mapper.
Contract-aware route binder types and helpers.