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.cpp

@@ -63,6 +63,15 @@ ParamSpec ParamSpec::makeChoice(std::string key, std::string label, std::string
     return spec;
 }
 
+ParamSpec ParamSpec::makePath(std::string key, std::string label, std::string help) {
+    ParamSpec spec;
+    spec.type = Type::Path;
+    spec.key = std::move(key);
+    spec.label = std::move(label);
+    spec.help = std::move(help);
+    return spec;
+}
+
 namespace {
 
 const ParamValue* find(const ParamValues& values, const std::string& key) {
@@ -99,6 +108,11 @@ 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) {
+    return paramText(values, key, fallback);
+}
+
+std::string paramText(const ParamValues& values, const std::string& key,
+                      const std::string& fallback) {
     if (const auto* value = find(values, key)) {
         if (const auto* v = std::get_if<std::string>(value)) return *v;
     }