Investigate DeviceIdentityStore minting under contention #526

While running the FileSystem unit-test assembly six times concurrently to reproduce a CI flake, one run had DeviceIdentityStoreTests.A_reader_arriving_while_the_name_is_rewritten_never_mints_over_it fail with a second device id minted, and PackagedAppDataTests.What_the_person_kept_beside_the_backlog_stays_where_it_was report Failed instead of Adopted. The first is the store's core "never mint over the id on disk" guarantee, so it may be a product bug rather than test noise. This session should reproduce under load and decide.

Context

Two tests in tests/Backlog.Infrastructure.FileSystem.UnitTests failed once in six concurrent Release runs of the whole assembly (each process on its own GUID temp root, so not a cross-process path collision):

1. DeviceIdentityStoreTests.A_reader_arriving_while_the_name_is_rewritten_never_mints_over_it (DeviceIdentityStoreTests.cs ~line 129): Assert.Equal([original], ids.Distinct()) saw two distinct GUIDs. The test runs Parallel.For(0, 100) constructing DeviceIdentityStore(IdentityPath, alternating names) and asserts every reader comes away with the one id on disk. A second id means a reader saw a half-written or empty file and minted over it ΓÇö the exact failure the store (src/Infrastructure/Backlog.Infrastructure.FileSystem, DeviceIdentityStore) is documented to prevent. Investigate whether the rewrite path is genuinely non-atomic under contention (e.g. File.WriteAllText truncate-then-write without a temp-file + File.Move/Replace) and fix in the store if so, with a regression test that fails reliably under load.

2. PackagedAppDataTests.What_the_person_kept_beside_the_backlog_stays_where_it_was (PackagedAppDataTests.cs ~line 98): expected Adopted, got Failed. Lower confidence; check whether it depends on timing or on disk state under load.

Reproduce with concurrent full-assembly runs (memory note: sequential repeats pass on unfixed code; use three or more concurrent runs of the built test exe with stdin closed and a timeout). This repository's CLAUDE.md orchestration gate applies to any write under src/ or tests/ ΓÇö route through orch-bug.

Filed automatically from a Claude Code suggested task.