fix: Relocate custom-option ids to a contiguous block, not by old-id value

create_custom_trees relocated an object's existing custom-option trees to
new ids computed as old_id + offset + 1, where offset is one above the
store's current maximum id. When the object already held the store's
largest id -- the normal case when re-styling the retained result of a
previous .opts() call -- the new id was roughly twice the maximum, so the
maximum id doubled on every re-customization and grew exponentially.

Ids are opaque keys into Store._custom_options; their absolute value
carries no meaning. The relocation only needs new ids disjoint from
existing keys and a bijection over the object's own id set. Assigning a
fresh contiguous block above the offset (indexed by position) satisfies
both and keeps the maximum id linear in the number of live customizations,
avoiding the unbounded bignum growth and the Python 3.11 4300-digit
integer-to-string ValueError it eventually triggered.

Author: SimonHeybrock

holoviews/core/options.py

@@ -1754,18 +1754,23 @@ def create_custom_trees(cls, obj, options, backend=None):
                 }
 
         custom_options = Store.custom_options(backend=backend)
-        for tree_id in obj_ids:
+        # Relocate the object's trees to a fresh contiguous block above the
+        # offset. Ids are opaque keys into Store._custom_options; the only
+        # requirements are that the new ids are disjoint from existing keys
+        # (guaranteed by the offset) and that distinct old ids map to distinct
+        # new ids. Indexing by position keeps the maximum id linear in the
+        # number of live customizations; deriving it from the old id's value
+        # (tree_id + offset + 1) doubles the maximum on every re-customization.
+        for new_id, tree_id in enumerate(obj_ids, start=offset):
             if tree_id is not None and tree_id in custom_options:
                 original = custom_options[tree_id]
                 clone = OptionTree(
                     items=original.items(), groups=original.groups, backend=original.backend
                 )
-                clones[tree_id + offset + 1] = clone
-                id_mapping.append((tree_id, tree_id + offset + 1))
             else:
                 clone = OptionTree(groups=available_options.groups, backend=backend)
-                clones[offset] = clone
-                id_mapping.append((tree_id, offset))
+            clones[new_id] = clone
+            id_mapping.append((tree_id, new_id))
 
             # Nodes needed to ensure allowed_keywords is respected
             for obj_type, opts in used_options.items():