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>

CLAUDE.md

@@ -48,6 +48,12 @@ add a `ParamSpec` instead. Values travel as `ParamValues` (a
 `map<string, variant<long,double,bool,string>>`), read back with
 `paramInt`/`paramDouble`/`paramBool`/`paramChoice`.
 
+Whether an effect works from the loaded photographs is part of the same
+contract: `Effect::needsSources()` (default true, false for blur). Only the
+first stage ever sees the sources, so `chainNeedsSources` (`src/core/chain.cpp`)
+just asks the first stage — a chain led by blur renders with no images loaded at
+all, and both front-ends gate on that rather than on "are there any images".
+
 Adding an effect: new `src/effects/<name>.{hpp,cpp}` subclassing `Effect`, one
 line in the registry list in `src/core/registry.cpp`, one line in the
 `effector_core` sources in `CMakeLists.txt`. Nothing else — it shows up in the