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/style.cpp

@@ -0,0 +1,39 @@
+#include "core/style.hpp"
+
+#include <algorithm>
+
+#include "core/registry.hpp"
+
+namespace effector {
+
+std::vector<ParamSpec> styleSpecs(const std::string& kind) {
+    std::vector<ParamSpec> specs;
+    if (kind.empty()) return specs;
+    for (const auto& effect : effects()) {
+        if (effect->styleKind() != kind) continue;
+        for (const auto& spec : effect->params()) {
+            if (!spec.inStyle) continue;
+            const bool known = std::any_of(specs.begin(), specs.end(), [&](const ParamSpec& seen) {
+                return seen.key == spec.key;
+            });
+            if (!known) specs.push_back(spec);
+        }
+    }
+    return specs;
+}
+
+ParamValues styleDefaults(const std::string& kind) {
+    ParamValues values;
+    for (const auto& spec : styleSpecs(kind)) values[spec.key] = paramDefault(spec);
+    return values;
+}
+
+const Style* findStyle(const StyleSet& styles, const std::string& kind, const std::string& name) {
+    if (kind.empty() || name.empty()) return nullptr;
+    for (const auto& style : styles) {
+        if (style.kind == kind && style.name == name) return &style;
+    }
+    return nullptr;
+}
+
+}  // namespace effector