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. Non-object bodies (text, arrays, scalars)
are excluded. Routes that need headers or non-object bodies declare an
explicit input mapper.
Contract-aware route binder types and helpers.