[C3] Bundle-flip-while-running test

[radical]

Track C validation. Depends on B5 (bundle install-id + atomic flip).

Test that a long-running aspire run survives a concurrent aspire update --self and bundle reparse flip:

1. Start a long-running CLI command (mock or real) that holds bundle file handles.
2. From another process, perform a bundle flip to a new version.
3. Assert: the running process completes without error; new shells observe the new bundle.

Windows-specific variant: assert atomicity of the flip — there must be no observable window where ~/.aspire/bundle/ doesn't exist or points at a partial dir.

Scenarios covered

S5.2, S5.3, S7.1, S7.2.

Manual companion

runbook-bundle-flip-race.md — Windows reparse non-atomic window repro (real Windows only).

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Part of: #29