fix: resolve conflict scan origins to the item that contains them

[iasakura]

The set membership tests in find_conflict_left always failed when the id being looked up pointed to an element other than the first one of its item. As test_insert_beside_multi_element_item_converges shows, when an origin_left_id points to such an element the scan can return a wrong insertion point, and replicas diverge depending on the order the updates were applied in.

This PR looks up the id of the item holding that id before testing membership. The lookup needs the client's node list, which this function does not have, so it comes in as the head_of parameter: DocStore passes get_node_index and read::resolve passes locate.

The loop also stops in a different place than yjs and yrs: where both of them break out, this scan keeps walking. Although that does not look like it affects convergence, I changed it to match yjs and yrs, as it only adds iterations with no benefit.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.97%. Comparing base (70873e4) to head (cab1585).
⚠️ Report is 2 commits behind head on main.
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Additional details and impacted files

@@             Coverage Diff             @@
##             main      #61       +/-   ##
===========================================
+ Coverage   15.29%   78.97%   +63.68%     
===========================================
  Files           1       59       +58     
  Lines         582    10841    +10259     
===========================================
+ Hits           89     8562     +8473     
- Misses        493     2279     +1786     

Flag	Coverage Δ
node-binding-test	15.29% <ø> (ø)

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