[P2] Improve e2e fixture isolation

[Gupta91]

Problem

E2E tests mutate .diffs, local storage, and tracked fixture files, which makes parallelization and debugging harder.

Expected Behavior

Add stronger cleanup/seeding helpers so tests can run independently and predictably.

Acceptance Criteria

• Shared helpers isolate .diffs and local storage test data.
• Tests clean up tracked fixture changes reliably.
• The suite is easier to parallelize later.