Marketplace: validate tree entry names and cap depth/manifest size

Fix round 1 from PR review of the tree/catalog parsing:

- collect_dir now rejects an entry whose name is ".", "..", empty, or
  contains "/", "\" or NUL before it becomes part of an item's rel_path —
  a crafted git tree could otherwise walk a file outside the item's own
  folder once that path is joined against the item root downstream.
- collect_dir caps recursion at 32 directory levels and counts
  directories (not just files) toward MAX_ITEM_FILES, so a tree that is
  wide or deep rather than merely file-heavy is still bounded.
- hook.json and plugins/.claude-plugin/marketplace.json are now rejected
  unparsed above 1 MiB, rather than handed to serde_json regardless of
  size.

A pre-read size query (checking a blob's size before reading it) is
deferred per controller ruling — this round reads the blob and checks
its length before parsing, which is enough for the JSON-parsing DoS
shape being closed here.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/src-tauri/src/marketplace/tree.rs

@@ -92,6 +92,29 @@ impl MemTree {
         self
     }
 
+    /// Place a file so that, inside `dir`, it is listed under the literal
+    /// entry name `name` — including a name `file`/`exec_file`/`symlink`
+    /// could never be asked to produce because it doesn't correspond to any
+    /// real filesystem path a caller here would construct: `.`, `..`, empty,
+    /// or containing `/`, `\` or a NUL byte. Exists only so a test can drive
+    /// `catalog::collect_dir`'s hostile-entry-name rejection without relying
+    /// on incidental behaviour of path-string splitting.
+    pub fn raw_named_file(mut self, dir: &str, name: &str, contents: &str) -> Self {
+        let path = if dir.is_empty() {
+            name.to_string()
+        } else {
+            format!("{}/{}", dir, name)
+        };
+        self.nodes.insert(
+            path,
+            MemNode::File {
+                data: contents.as_bytes().to_vec(),
+                executable: false,
+            },
+        );
+        self
+    }
+
     fn is_dir(&self, path: &str) -> bool {
         if path.is_empty() {
             return true;