feat(plugins): introduce plugin API 20 for new ui surface

ui.markdown and the new input submitOnEnter and scroll stickToBottom,
onScroll and scrollToBottomRev props expand the plugin UI contract.
Plugins that rely on them can declare plugin_api = 20, so an older
shell rejects them at load instead of failing at runtime. Declaration
only, matching the API 18/19 precedent; the manifest range check in
plugin_registry enforces it.

Assisted-by: Fable 5 (main session), claude-code v2.1.217

Author: gegnep

src/scripting/plugin_api.h

@@ -19,7 +19,8 @@ namespace scripting {
   inline constexpr std::uint32_t kServiceLifecyclePluginApiVersion = 17;
   inline constexpr std::uint32_t kPanelFrameTickPluginApiVersion = 18;
   inline constexpr std::uint32_t kFormatTimeTimezonePluginApiVersion = 19;
-  inline constexpr std::uint32_t kCurrentPluginApiVersion = kFormatTimeTimezonePluginApiVersion;
+  inline constexpr std::uint32_t kPluginUiPropsPluginApiVersion = 20;
+  inline constexpr std::uint32_t kCurrentPluginApiVersion = kPluginUiPropsPluginApiVersion;
 
   static_assert(kOldestSupportedPluginApiVersion <= kCurrentPluginApiVersion);