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>

tests/core_tests.cpp

@@ -194,13 +194,12 @@ int main() {
         const auto pinnedAgain = renderBlur(sources, 300, 200, 6, {{"first_source", true}});
         check(sameBytes(pinned, pinnedAgain), "first_source is reproducible");
 
-        bool threw = false;
-        try {
-            renderBlur({}, 100, 100, 1, {});
-        } catch (const EffectError&) {
-            threw = true;
-        }
-        check(threw, "no sources throws");
+        // Blur declares that it needs no sources, so with none it softens the
+        // empty canvas it starts from instead of refusing to run.
+        const auto blank = renderBlur({}, 100, 100, 1, {});
+        bool allBlack = blank.width() == 100 && blank.height() == 100;
+        for (std::size_t i = 0; i < blank.byteSize(); ++i) allBlack = allBlack && blank.data()[i] == 0;
+        check(allBlack, "without sources blur renders an empty canvas");
     }
 
     std::cout << "chain\n";
@@ -241,6 +240,29 @@ int main() {
             threw = true;
         }
         check(threw, "an empty chain throws");
+
+        // Sources are an effect's own business: scanlines cuts strips out of
+        // them, blur only softens what it is handed and runs without any.
+        check(effectByName("scanlines")->needsSources(), "scanlines needs source images");
+        check(!effectByName("blur")->needsSources(), "blur needs no source images");
+        check(chainNeedsSources({scanlines, blur}), "a chain led by scanlines needs sources");
+        check(!chainNeedsSources({blur, scanlines}),
+              "a chain led by blur needs none, later stages see the stage before them");
+        check(!chainNeedsSources({}), "an empty chain needs no sources");
+
+        Rng bare(5);
+        const auto sourceless = renderChain({blur}, {}, 200, 150, bare);
+        check(sourceless.width() == 200 && sourceless.height() == 150,
+              "blur renders a canvas without sources");
+
+        bool scanlinesThrew = false;
+        try {
+            Rng rng(1);
+            renderChain({scanlines}, {}, 100, 100, rng);
+        } catch (const EffectError&) {
+            scanlinesThrew = true;
+        }
+        check(scanlinesThrew, "scanlines still refuses to run without sources");
     }
 
     std::cout << "project\n";