Add iree_status_allocate_copy() which copies both file and message into
trailing storage, producing self-contained statuses with no dangling
pointers. This is needed for deserialized statuses where the source
buffer is transient.

Also add public APIs for inspecting status internals:
- iree_status_source_location(): extract file:line from a status
- iree_status_message(): extract the primary message string
- iree_status_payload_visitor_fn_t: callback for iterating payloads
- iree_status_payload_type_t enum: typed payload classification

Move iree_status_payload_t definition from status_payload.h to status.h
so the visitor callback can reference it in the public API.

Co-Authored-By: Claude <[email protected]>