test(fixtures): add reusable temp paths helper

[vernonstinebaker]

Summary

• add a small shared src/test_helpers.zig fixture with TempPaths, which creates an isolated temporary NullHub root backed by std.testing.tmpDir
• use the new fixture in existing backend tests that previously relied on hardcoded /tmp/... paths and manual cleanup
• adopt the fixture in real tests across three subsystems: supervisor runtime-state persistence, supervisor manager log/restart behavior, and installer directory creation
• keep the helper intentionally narrow so it unlocks later testing work without introducing a broad test framework

Validation

• zig build test -Dembed-ui=false -Dbuild-ui=false --summary all
• npm --prefix ui ci --no-audit --no-fund
• npm --prefix ui run build
• bash tests/test_e2e.sh
• zig fmt --check src/
• git diff --check

Notes

• this PR focuses only on temporary NullHub root/path isolation; it does not yet add fake upstream HTTP helpers or broader integration harnesses
• the immediate benefit is removal of several hardcoded /tmp test roots and more reliable cleanup semantics for future TDD-style backend tests
• a natural follow-up is extending the fixture layer with a tiny fake upstream helper for orchestration and installer/update integration tests