`{.ffiEvent.}` previously accepted exactly one parameter, forcing every
multi-field event to declare a hand-written {.ffi.} payload type. The macro
now bundles two or more parameters into a synthesised, registered envelope
object named `<WireNamePascalCase>Payload`, whose fields are the parameters,
and dispatches an instance of it. A single parameter still rides the wire
directly (scalar or existing {.ffi.} object), so this is backwards
compatible. Because the envelope is registered like any {.ffi.} type, the
foreign bindings gain it as a first-class struct plus a typed handler.
The timer example gains an `on_job_scheduled(jobId, willRunCount)` event to
exercise the path; the C++ and Rust bindings are regenerated accordingly.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
