Materializer crashes when UpdatedRecord changes a primary key column

[alco]

Bug Description

When a row's primary key column is updated in the database, the Materializer crashes with a KeyError because it tries to look up the new key (post-update) in an index that only contains the old key (pre-update).

Reproduction

Given a table where a column is part of the primary key (e.g., task_user_acl with PK (organization_id, subject_user_id, task_id)):

UPDATE task_user_acl SET task_id = 'new-task-id' WHERE task_id = 'old-task-id';

This causes the Materializer to crash:

** (KeyError) key "\"public\".\"task_user_acl\"/\"org-092\"/\"user-0078\"/\"org-100-task-1109\"" not found in:

%{
  "\"public\".\"task_user_acl\"/\"org-092\"/\"user-0078\"/\"org-092-task-0013\"" => "org-092-task-0013",
  ...
}

    :erlang.map_get("...new_key...", %{"...old_key..." => ...})
    (electric) lib/electric/shapes/consumer/materializer.ex:466: anonymous fn/3 in Electric.Shapes.Consumer.Materializer.apply_changes/2

Root Cause

In materializer.ex, the apply_changes/2 function handles UpdatedRecord by extracting only key (the new key after the update):

%Changes.UpdatedRecord{
  key: key,
  record: record,
  ...
},

Then it uses key to look up the existing value:

old_value = Map.fetch!(index, key)

But when a PK column changes, key != old_key. The index contains entries keyed by old_key, so the lookup fails.

Fix

The fix requires:

1. Extract old_key from UpdatedRecord in addition to key
2. Handle the case where old_key is nil (PK didn't change) by defaulting to key
3. Use old_key when looking up/removing from the index
4. Use old_key when removing from tag indices
5. Use key (new key) when inserting into the index and adding to tag indices

 %Changes.UpdatedRecord{
   key: key,
+  old_key: old_key,
   record: record,
   move_tags: move_tags,
   removed_move_tags: removed_move_tags
 },
 {{index, tag_indices}, counts_and_events} ->
+  old_key = old_key || key
   ...
   tag_indices =
     tag_indices
-    |> remove_row_from_tag_indices(key, removed_move_tags)
+    |> remove_row_from_tag_indices(old_key, removed_move_tags)
     |> add_row_to_tag_indices(key, move_tags)

   if columns_present do
     {value, original_string} = cast!(record, state)
-    old_value = Map.fetch!(index, key)
+    {old_value, index} = Map.pop!(index, old_key)
     index = Map.put(index, key, value)

Context

This affects shapes that use subqueries with ACL tables where the ACL table has a composite primary key that includes a foreign key column. When that FK column is updated, the shape's materializer crashes.