feat(ui): tree-owned scroll gestures and geometry hit-testing

Drag, flick and tap-vs-scroll now live on the scrollX/scrollY flag in
ui.lua, so any scroll box pans with no app code. tree.hit returns the
deepest node by geometry and dispatch bubbles to the nearest handler,
dropping the now-unused CAPTURE flag. keyboard moves in as ui.keyboard,
and embed compiles nested lib dirs to dotted module names.

tools/embed.py

@@ -35,7 +35,9 @@ def main():
     lib_dir = pathlib.Path(lib_dir)
     out_path = pathlib.Path(out_path)
 
-    sources = sorted(lib_dir.glob("*.lua"))
+    # Recurse so ui/keyboard.lua embeds as the module require("ui.keyboard") asks for: the
+    # module name is the path with dots, the C array identifier the same with underscores.
+    sources = sorted(lib_dir.rglob("*.lua"))
     arrays = []
     rows = []
     for src in sources:
@@ -45,10 +47,12 @@ def main():
         subprocess.run([luac32, str(src), tmp_path], check=True)
         bytecode = pathlib.Path(tmp_path).read_bytes()
         pathlib.Path(tmp_path).unlink()
-        name = src.stem
+        parts = src.relative_to(lib_dir).with_suffix("").parts
+        name = ".".join(parts)
+        ident = "_".join(parts)
         comma = ", ".join(f"0x{b:02x}" for b in bytecode)
-        arrays.append(f"static const unsigned char {name}[] = {{{comma}}};\n")
-        rows.append(f'    {{"{name}", {name}, sizeof({name})}},')
+        arrays.append(f"static const unsigned char {ident}[] = {{{comma}}};\n")
+        rows.append(f'    {{"{name}", {ident}, sizeof({ident})}},')
 
     footer = "const EmbeddedModule embedded_modules[] = {{\n{rows}\n}};\nconst size_t embedded_modules_count = {count};\n"
     generated = HEADER + "".join(arrays) + footer.format(rows="\n".join(rows), count=len(sources)) + FOOTER