Add external/virtual tree path tests in separate file

Split tests to avoid exceeding file size limit:
- doublets/tests/store_internals.rs: Original tests (827 lines)
- doublets/tests/external_tree_tests.rs: New tests (436 lines)

New tests (17 tests in external_tree_tests.rs):
| Test Name | Purpose |
|-----------|---------|
| `split_virtual_source_operations` | Test virtual source handling |
| `split_external_source_tree_iteration` | Test tree iteration with handler |
| `split_virtual_target_operations` | Test virtual target handling |
| `split_search_all_tree_paths` | Test search across tree branches |
| `split_count_usages_tree_branches` | Test count_usages tree traversal |
| `split_tree_ordering_operations` | Test tree ordering comparisons |
| `split_each_usages_core_branches` | Test recursive iteration |
| `unit_virtual_source_operations` | Unit store virtual source tests |
| `unit_tree_ordering_operations` | Unit store ordering tests |
| `unit_each_usages_core_branches` | Unit store iteration tests |
| `split_count_links_two_element_query` | Test 2-element query counting |
| `split_is_virtual_detection` | Test is_unused detection |
| `split_resolve_dangling_operations` | Test dangling link resolution |

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

Author: konard