Slice 10 a1: preview pipeline spike — wlr pixels -> RMLUi texture (Fork-B GO)

The keystone for the stage dock. Proves Fork B on the real target (Mesa crocus,
HD 4400): a toplevel's pixels, rendered by the wlr GLES2 renderer into a LINEAR
ARGB8888 dmabuf, import as an EGLImage -> sampled GL texture in the sibling RMLUi
GLES 3.2 context (the slice-3 bridge run in reverse) and composite into an
<img src="unbox-preview://N"> inside a ui surface — upright, color-correct.

Public surface (ui.hpp): class Preview (source_uri/source_width/source_height/
refresh) + UiSubstrate::create_preview(wlr_scene_tree*) -> unique_ptr<Preview>
(nullptr if no GL path; never throws). Kernel-suite probes: ui_preview_import_is_dmabuf,
ui_pixel(x,y). Clean four-resource teardown (URI reg, GL texture, EGLImage, dmabuf);
refresh-after-source-destruction is UB (consumer drops Preview on unmap).

kernel 42 cases/150 assertions green on build + build-asan (asan clean, no new
suppressions). Edits confined to packages/kernel/.

1 files changed, 15 insertions, 0 deletions

diff --git a/packages/kernel/src/server.cpp b/packages/kernel/src/server.cpp
index a5f8365..b302c68 100644
--- a/packages/kernel/src/server.cpp
+++ b/packages/kernel/src/server.cpp
@@ -81,6 +81,14 @@ auto Server::ui_fence_sync_active() const -> bool {
     return impl_->substrate != nullptr && impl_->substrate->fence_sync_active();
 }
 
+auto Server::ui_preview_import_is_dmabuf() const -> bool {
+    return impl_->substrate != nullptr && impl_->substrate->preview_import_is_dmabuf();
+}
+
+auto Server::ui_pixel(int x, int y) const -> unsigned int {
+    return impl_->substrate != nullptr ? impl_->substrate->surface_pixel(x, y) : 0U;
+}
+
 void Server::ui_set_touch_override(UiTouchOverride ov) {
     if (impl_->substrate == nullptr) {
         return;
@@ -104,6 +112,13 @@ auto PerExtensionUi::create_surface(const UiSurfaceSpec& spec) -> std::unique_pt
     return server_->substrate->create_surface(id_, parent, spec);
 }
 
+auto PerExtensionUi::create_preview(wlr_scene_tree* source) -> std::unique_ptr<Preview> {
+    if (server_->substrate == nullptr) {
+        return nullptr;
+    }
+    return server_->substrate->create_preview(source);
+}
+
 auto PerExtensionUi::available() const -> bool {
     return server_->substrate != nullptr && server_->substrate->available();
 }