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/effects/blur.hpp

@@ -4,17 +4,19 @@
 
 namespace effector {
 
-// Fills the canvas with one of the sources and softens it with a separable box
-// blur, repeated a few times to approximate a gaussian.
+// Softens its input with a separable box blur, repeated a few times to
+// approximate a gaussian. It needs no source images of its own: given none it
+// blurs an empty canvas, which is what makes it usable as a pure filter stage.
 class BlurEffect : public Effect {
 public:
     std::string name() const override { return "blur"; }
     std::string title() const override { return "Blur"; }
     std::string description() const override {
-        return "Fills the canvas from a randomly chosen source and softens it with a repeated "
-               "box blur, horizontally, vertically or both.";
+        return "Softens its input with a repeated box blur, horizontally, vertically or both; "
+               "with sources loaded it fills the canvas from a randomly chosen one first.";
     }
     const std::vector<ParamSpec>& params() const override;
+    bool needsSources() const override { return false; }
 
     Image render(const std::vector<Image>& sources, int width, int height, Rng& rng,
                  const ParamValues& values) const override;