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>

CLAUDE.md

@@ -41,10 +41,12 @@ env -u WAYLAND_DISPLAY GDK_BACKEND=x11 DISPLAY=:79 build/effector ...
 
 `Effect` (`src/core/effect.hpp`) exposes `params()` returning `std::vector<ParamSpec>`
 (`src/core/params.hpp`). Those specs are the *only* thing the GUI and CLI know
-about an effect: `src/gui/window.cpp` builds SpinButtons/Switches/DropDowns from
-them and `src/cli/options.cpp` generates flags from them (`strip_height` →
-`--strip-height`). Never add effect-specific branches to the GUI or the parser —
-add a `ParamSpec` instead. Values travel as `ParamValues` (a
+about an effect: `src/gui/window.cpp` builds SpinButtons/Switches/DropDowns/file
+choosers from them and `src/cli/options.cpp` generates flags from them
+(`strip_height` → `--strip-height`). Never add effect-specific branches to the
+GUI or the parser — add a `ParamSpec` instead. A `Path` param names a file on
+disk; like a source path it is written relative to the project folder. Values
+travel as `ParamValues` (a
 `map<string, variant<long,double,bool,string>>`), read back with
 `paramInt`/`paramDouble`/`paramBool`/`paramChoice`.