M2.6: say why pruning reads a form's /Subtype only when direct

The sweep flags the read in Pruner::form, moved there by this change. A /Subtype given by reference reads as no form wherever the workspace tells forms from images; resolving it is a change across all of them, and the comment says so.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AmsdeUCdTaD72jkDw2pJKY

crates/organize/src/prune.rs

@@ -222,6 +222,9 @@ impl Pruner<'_> {
     fn form(&mut self, xobjects: &Dict, name: &[u8], resources: &Dict, owner: Owner, depth: u8) {
         let Some(r) = xobjects.get(name).and_then(Object::as_ref) else { return };
         let Object::Stream(s) = &*self.doc.get(r) else { return };
+        // A `/Subtype` given by reference reads as no form here, as wherever the workspace tells
+        // forms from images (redaction, edit and Optimize among them): resolving it is a change
+        // across all of them.
         if s.dict.name(b"Subtype") != Some(b"Form") {
             return;
         }