Let several text stages share one named look

A picture with a title, a caption and a credit meant typing the same
font, size and colour into every text stage, and changing the look meant
typing them all again. A project now holds named styles: a stage follows
one, and editing it moves every stage that follows it at once.

The front-ends learn nothing effect-specific. A ParamSpec can be marked
.styled(), saying a style carries it rather than the stage, and
Effect::styleKind() names the pool it comes from -- so styleSpecs(kind)
is to a style what params() is to a stage, and a second text-ish effect
would slot into the styles that already exist. The look is what a style
carries; the words and the placement stay on the stage, which is what
lets two stages share a typography in two different corners.

stageValues() resolves one in a single place, and a stage keeps a full
set of values of its own beside the style, so switching it to Custom,
dropping the style, or opening the project in a build that does not know
it renders exactly the same picture. A style moves stages together; it
never holds the only copy of anything.

The command line builds no styles and passes none, so it renders what it
always did.

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

src/core/project.hpp

@@ -8,6 +8,7 @@
 #include "core/canvas.hpp"
 #include "core/chain.hpp"
 #include "core/params.hpp"
+#include "core/style.hpp"
 
 namespace effector {
 
@@ -17,6 +18,10 @@ namespace effector {
 struct Project {
     std::vector<std::string> sources;  // absolute paths
     std::vector<ChainStage> stages;
+    // The named looks the stages draw their formatting from. Kept beside the
+    // chain rather than inside it because that is the whole point of a style:
+    // several stages share one, and editing it moves all of them at once.
+    StyleSet styles;
     int width = 1920;
     int height = 1080;
     // The named format the canvas came from, empty for a custom pixel size.
@@ -36,7 +41,9 @@ std::string projectToJson(const Project& project, const std::string& directory =
 
 // Throws EffectError on malformed JSON, an unknown effect or a missing chain.
 // Relative source paths are resolved against `directory`; params the effect
-// does not declare are dropped, and missing ones keep the effect's default.
+// does not declare are dropped, and missing ones keep the effect's default. A
+// style of an unknown kind, or a stage naming a style the file does not hold,
+// is dropped the same way: the stage keeps its own values and still renders.
 Project projectFromJson(const std::string& text, const std::string& directory = {});
 
 // Both throw EffectError: the file is the user's, so failures are reported