Let effects declare whether they need source images

Whether an effect works from the loaded photographs is now part of the
declarative plug-in contract, next to params(): Effect::needsSources()
defaults to true, and blur — which only softens what it is handed — says
no and blurs an empty canvas when given nothing.

Only the first stage of a cascade ever sees the sources, so
chainNeedsSources() just asks the first stage. Both front-ends gate on
that instead of on "are there any images": the CLI only refuses an empty
batch when the chain wants one, and the interface renders a blur-led
chain with no images loaded, saying whichever thing is actually missing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/effect.hpp

@@ -27,6 +27,11 @@ public:
     virtual std::string description() const = 0;
     virtual const std::vector<ParamSpec>& params() const = 0;
 
+    // Whether the effect builds its image out of the loaded source images.
+    // Effects that only transform what they are handed (blur) say no, and a
+    // chain that starts with one of them needs no sources at all.
+    virtual bool needsSources() const { return true; }
+
     virtual Image render(const std::vector<Image>& sources, int width, int height, Rng& rng,
                          const ParamValues& values) const = 0;