refactor: Fix compilation warnings across multiple crates

- Remove unused imports (Filter, Digest, Resource, etc.)
- Prefix unused variables with underscore (_pk, _size)
- Fix snake_case naming for local variables
- Remove unused ArgumentError enum in pmoupnp
- Apply cargo fix suggestions for unused imports

Remaining warnings are async_fn_in_trait style warnings which
would require API breaking changes to address.

pmoupnp/src/actions/action_instance.rs

@@ -1,10 +1,8 @@
 use std::{
     collections::{HashMap, HashSet},
-    env::var,
     sync::Arc,
 };
 
-use bevy_reflect::Reflect;
 use xmltree::{Element, XMLNode};
 
 use crate::actions::{Action, ActionData, ActionInstance, ArgInstanceSet};