Add deterministic coverage for concurrent index_router cache insertion

[coderabbitai]

Summary

Codecov coverage for phlex/core/index_router.cpp Line 366 is nondeterministic. The affected code is the if (not inserted) return path in multilayer_slots_for.

Concurrent calls can make inserted either true or false. This produces small, unrelated coverage changes in pull requests.

Required change

Add a focused multithreaded test that deterministically exercises the contended-insert path. Use synchronization, such as a barrier, so multiple threads call multilayer_slots_for with the same layer_hash at the same time.

Do not change the cursor or GSL work in the current pull request for this task.

Rationale

The branch is valid, but its coverage depends on thread scheduling. A deterministic test will reduce Codecov noise and verify the concurrent cache behavior.

Affected area

• phlex/core/index_router.cpp
• Tests for multilayer_slots_for and concurrent index-router behavior

Acceptance criteria

• The test forces one or more callers through the not inserted branch.
• The test is stable across repeated runs.
• Coverage consistently includes the return path at phlex/core/index_router.cpp Line 366.
• The test verifies that concurrent callers receive consistent slot and end-token-entry results.

References

• Pull request: refactor(core): adopt gsl::not_null where appropriate #839
• Discussion: refactor(core): adopt gsl::not_null where appropriate #839 (comment)
• Requested by: @knoepfel