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/params.hpp

@@ -24,9 +24,17 @@ struct ParamSpec {
     int digits = 2;
     std::string defaultChoice;
     std::vector<std::string> choices;
+    // Whether a named style carries this param rather than the stage it sits
+    // on: see core/style.hpp.
+    bool inStyle = false;
 
     std::string flag() const;  // "strip_height" -> "--strip-height"
 
+    // A copy of this spec marked as carried by a style, so that a declaration
+    // reads makeDouble("font_size", ...).styled() and the factories keep the
+    // signatures they have.
+    ParamSpec styled() const;
+
     static ParamSpec makeInt(std::string key, std::string label, long defaultValue, long minimum,
                              long maximum, std::string help = {}, long step = 1);
     static ParamSpec makeDouble(std::string key, std::string label, double defaultValue,
@@ -53,6 +61,10 @@ struct ParamSpec {
 using ParamValue = std::variant<long, double, bool, std::string>;
 using ParamValues = std::map<std::string, ParamValue>;
 
+// The value a spec stands for when nothing has been set: one place, so that
+// an effect's defaults and a style's are filled the same way.
+ParamValue paramDefault(const ParamSpec& spec);
+
 long paramInt(const ParamValues& values, const std::string& key, long fallback);
 double paramDouble(const ParamValues& values, const std::string& key, double fallback);
 bool paramBool(const ParamValues& values, const std::string& key, bool fallback);