[ty] Shrink size of AstNodeRef (#20028)

## Summary

Removes the `module_ptr` field from `AstNodeRef` in release mode, and
change `NodeIndex` to a `NonZeroU32` to reduce the size of
`Option<AstNodeRef<_>>` fields.

I believe CI runs in debug mode, so this won't show up in the memory
report, but this reduces memory by ~2% in release mode.

crates/ruff_python_formatter/src/comments/debug.rs

@@ -196,12 +196,12 @@ mod tests {
     fn debug() {
         let continue_statement = StmtContinue {
             range: TextRange::new(TextSize::new(18), TextSize::new(26)),
-            node_index: AtomicNodeIndex::dummy(),
+            node_index: AtomicNodeIndex::NONE,
         };
 
         let break_statement = StmtBreak {
             range: TextRange::new(TextSize::new(55), TextSize::new(60)),
-            node_index: AtomicNodeIndex::dummy(),
+            node_index: AtomicNodeIndex::NONE,
         };
 
         let source = r"# leading comment