Marker type — each modality returns its own value-object shape from
signalAt. Vision returns Light, smell returns Smell, sound
returns Sound, touch returns Touch. Callers narrow at the call
site. Per the requirements doc's "No premature substrate widening"
rule, no shared Signal base class.
Marker type — each modality returns its own value-object shape from
signalAt. Vision returnsLight, smell returnsSmell, sound returnsSound, touch returnsTouch. Callers narrow at the call site. Per the requirements doc's "No premature substrate widening" rule, no sharedSignalbase class.