add core view extension with JTE and Thymeleaf support

flash-extensions/flash-ext-view-thymeleaf/docs/handlers.md

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+# Handlers
+
+Use `ThymeleafHandler` for class-based Thymeleaf routes.
+
+## Lifecycle
+
+- `onViewInit()` runs once at boot.
+- `render(...)` runs per request.
+
+Use `onViewInit()` to cache dependencies via `require(...)`.
+
+## Example
+
+```java
+@GET("/dashboard")
+@Template("pages/dashboard")
+public final class DashboardPage extends ThymeleafHandler {
+
+    private DashboardService service;
+
+    @Override
+    protected void onViewInit() {
+        service = require(DashboardService.class);
+    }
+
+    @Override
+    public ViewModel render(dev.relism.models.Request req) {
+        return ViewModel.empty()
+            .with("title", "Dashboard")
+            .with("stats", service.stats());
+    }
+}
+```
+
+Use `render(Request, Response)` when you need response access while building model variables.