fix: add weakref callback to clean up resolve cache entries

The cache used dict keyed by id(doc) with no cleanup, causing a slow
memory leak in long-running processes. Added a weakref finalizer
callback that removes the cache entry when the doc is garbage
collected.

Fixes ISSUES.md #27.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sciyoshi

prosemirror/model/resolvedpos.py

@@ -228,7 +228,11 @@ def resolve_cached(cls, doc: "Node", pos: int) -> "ResolvedPos":
 
 class _ResolveCache:
     def __init__(self, doc: "Node") -> None:
-        self.doc_ref = weakref.ref(doc)
+        doc_id = id(doc)
+        self.doc_ref = weakref.ref(
+            doc,
+            lambda _ref: _resolve_cache.pop(doc_id, None),
+        )
         self.elts: list[ResolvedPos | None] = [None] * _RESOLVE_CACHE_SIZE
         self.i = 0