A signature collision is a fault only when at least one colliding source
carries an unaliased param slot — the configuration axis the author left
unnamed (sma_cross's length). Equal-signature sources with no param are
genuinely interchangeable (fan_composite's Pass/Pass, add(price,price)) and
stay legal; the render uses the positional letter for them. Keeps the
construction blast radius to the param-bearing alias-less fixtures.
Refs #44
Brummel
22a690361a