add core view extension with JTE and Thymeleaf support

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

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+# Architecture
+
+`flash-ext-view-thymeleaf` has two layers:
+
+1. **Boot-time**
+   - `ThymeleafExtension` installs runtime + annotation processor.
+   - `ThymeleafTargetResolver` validates handlers and resolves `@Template` / `@Fragment`.
+   - Resolved targets are cached per handler class.
+
+2. **Request-time**
+   - Handler builds local `ViewModel`.
+   - Runtime injects globals under reserved `global` namespace and merges local model.
+   - Thymeleaf renders template or fragment.
+
+## Handler Contract
+
+- Must extend `ThymeleafHandler`.
+- Must have route annotation (`@Route`, `@GET`, `@POST`, ...).
+- Must declare exactly one view annotation:
+  - `@Template`
+  - `@Fragment`
+
+Invalid configurations fail fast at startup.