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/font.sx

@@ -0,0 +1,20 @@
+#import "modules/std.sx";
+#import "modules/ui/types.sx";
+#import "modules/ui/glyph_cache.sx";
+
+// Global glyph cache pointer for views (Label, Button) to access
+g_font : *GlyphCache = null;
+
+set_global_font :: (font: *GlyphCache) {
+    g_font = font;
+}
+
+// Convenience measurement function for views
+measure_text :: (text: string, font_size: f32) -> Size {
+    if g_font == null {
+        // Fallback approximate measurement
+        scale := font_size / 16.0;
+        return Size.{ width = xx text.len * 8.0 * scale, height = font_size };
+    }
+    g_font.measure_text(text, font_size);
+}