updated README and a bunch of other stuff

parts/spacetime/src/lib.rs

@@ -0,0 +1,9 @@
+use mize::prelude::*;
+
+/// Stub SpacetimeDB client part.
+/// SDK wiring (connect with auth.client.get_id_token) comes later; dependents may fail until then.
+#[unsafe(no_mangle)]
+pub extern "C" fn part_spacetime() -> MizeResult<()> {
+    opt("url");
+    Ok(())
+}