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/chain.cpp

@@ -11,8 +11,26 @@ std::vector<const ChainStage*> enabledStages(const std::vector<ChainStage>& stag
     return running;
 }
 
+ParamValues stageValues(const ChainStage& stage, const StyleSet& styles) {
+    ParamValues values = stage.effect->defaults();
+    for (const auto& [key, value] : stage.values) values[key] = value;
+
+    const Style* style = findStyle(styles, stage.effect->styleKind(), stage.style);
+    if (style != nullptr) {
+        // Only the params the effect marked styled(): the words and the
+        // placement stay the stage's own, which is what lets two stages share
+        // one look without sharing a position.
+        for (const auto& spec : stage.effect->params()) {
+            if (!spec.inStyle) continue;
+            const auto found = style->values.find(spec.key);
+            if (found != style->values.end()) values[spec.key] = found->second;
+        }
+    }
+    return values;
+}
+
 Image renderChain(const std::vector<ChainStage>& stages, const std::vector<Image>& sources,
-                  int width, int height, Rng& rng) {
+                  int width, int height, Rng& rng, const StyleSet& styles) {
     if (stages.empty()) throw EffectError("The effect chain is empty.");
     for (const auto& stage : stages) {
         if (!stage.effect) throw EffectError("The effect chain has an empty stage.");
@@ -28,8 +46,7 @@ Image renderChain(const std::vector<ChainStage>& stages, const std::vector<Image
     for (std::size_t i = 0; i < running.size(); ++i) {
         const auto& stage = *running[i];
 
-        auto values = stage.effect->defaults();
-        for (const auto& [key, value] : stage.values) values[key] = value;
+        const auto values = stageValues(stage, styles);
 
         if (i == 0) {
             image = stage.effect->render(sources, width, height, rng, values);