Stop deterministic test directory allocation from deleting colliding roots

[flyingrobots]

Defect

Two deterministic filesystem-test allocators treat an existing candidate directory as disposable:

• crates/warp-core/src/causal_wal_tests.rs::deterministic_test_dir
• crates/warp-core/tests/causal_wal_hardening_tests.rs::temp_wal_root

On AlreadyExists, each calls remove_dir_all and recreates the path. The counter that chooses the suffix is process-local while the target directory is shared. Two overlapping test processes can therefore select the same candidate, and one process can delete the other process's live fixture.

The destructive collision behavior is present in the current code. A cross-process executable witness is still required before repair.

Acceptance

• Add a RED independent-process witness: process B owns a candidate directory and marker; process A collides with that candidate and must not remove, replace, or mutate B's material.
• An allocator claims only a path it created atomically.
• AlreadyExists never authorizes remove_dir_all without exact ownership evidence.
• Stale-fixture reclamation, if retained, is a separate bounded operation with an ownership marker and exact-root aperture.
• Parallel and overlapping test invocations cannot delete one another's fixtures.
• Preserve deterministic test naming where it remains useful; do not widen this into production filesystem authority.

[coderabbitai]

🔗 Related PRs

#598 - feat(warp-core): add filesystem WAL failure atomicity [merged]
#618 - Lock retained evidence crosswalk [merged]
#627 - test(warp-core): gate DIND durability convergence [merged]
#683 - Execute admitted data-only operations in Echo [merged]
#715 - Persist filesystem WAL writer epochs [merged]

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