Add an image overlay effect

Lays a single picture from disk over its input: fitted, filled, stretched,
left at its own size or tiled, centred or pinned to the top left corner,
nudged by an offset and mixed in by its opacity.

The file it needs is a new kind of param, Path, so the interface shows a
chooser and the CLI takes a file name, both without knowing an overlay
exists. Projects write it relative to their own folder, as they do a
source, and Image::load can now hand back the alpha channel instead of
flattening it onto white, so a cut-out PNG keeps its cut-out.

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

src/core/params.hpp

@@ -10,7 +10,7 @@ namespace effector {
 // Declarative description of one effect knob. The GUI turns these into widgets
 // and the CLI turns them into flags, so neither knows about any effect.
 struct ParamSpec {
-    enum class Type { Int, Double, Bool, Choice };
+    enum class Type { Int, Double, Bool, Choice, Path };
 
     Type type = Type::Int;
     std::string key;
@@ -35,6 +35,10 @@ struct ParamSpec {
                               std::string help = {});
     static ParamSpec makeChoice(std::string key, std::string label, std::string defaultChoice,
                                 std::vector<std::string> choices, std::string help = {});
+    // A path to a file on disk, empty by default: a chooser in the interface, a
+    // flag taking any string on the command line, written relative to the
+    // project folder so that a folder still opens after it is moved.
+    static ParamSpec makePath(std::string key, std::string label, std::string help = {});
 };
 
 using ParamValue = std::variant<long, double, bool, std::string>;
@@ -45,5 +49,7 @@ double paramDouble(const ParamValues& values, const std::string& key, double fal
 bool paramBool(const ParamValues& values, const std::string& key, bool fallback);
 std::string paramChoice(const ParamValues& values, const std::string& key,
                         const std::string& fallback);
+std::string paramText(const ParamValues& values, const std::string& key,
+                      const std::string& fallback);
 
 }  // namespace effector