ui: port game UI framework into library/modules/ui

20 files (~3,830 lines): view protocol, layout, renderer, glyph cache,
fonts, gestures, animation, scroll, stacks, modifiers, etc.

Internal imports rewritten from "ui/..." to "modules/ui/...".
Consumers now `#import "modules/ui"` from any project; no symlink
hacks needed. Verified by compiling game/main.sx without its local
ui/ — resolves via the Phase 6 stdlib fallback.

library/modules/ui/view.sx

@@ -0,0 +1,23 @@
+#import "modules/ui/types.sx";
+#import "modules/ui/render.sx";
+#import "modules/ui/events.sx";
+
+View :: protocol {
+    // Measure: given a size proposal, return desired size
+    size_that_fits :: (proposal: ProposedSize) -> Size;
+
+    // Place: position children within the given bounds
+    layout :: (bounds: Frame);
+
+    // Render: emit render nodes
+    render :: (ctx: *RenderContext, frame: Frame);
+
+    // Event handling: return true if the event was consumed
+    handle_event :: (event: *Event, frame: Frame) -> bool;
+}
+
+// A child view with its computed frame (set during layout)
+ViewChild :: struct {
+    view: View;
+    computed_frame: Frame = .zero();
+}