test(packing): rename single timestep regression tests

[lfengad]

Summary

• Rename packers_test.py to uses_single_timestep_test.py.
• Avoid a destination collision with the Cosmos-Framework release mapping, which now has a different source-side packers_test.py.
• Preserve the existing PR fix(packing): gate timestep-embedding fast path on timestep values, not tensor shape #236 regression coverage unchanged.

Validation

• Git detects a 100% rename with 0 insertions and 0 deletions.
• python3 -m py_compile passed for the renamed file.
• The standard uv test and Ruff commands were not run because uv is unavailable in the current environment.