fix: allow topic segmentation without precompression

[JiataiWang]

What broke

pre_compress and topic_segment are documented as independent options, but enabling topic segmentation without pre-compression crashes during LightMemory initialization because self.compressor is only assigned inside the pre-compression branch and then passed unconditionally to the segmenter factory.

Fix

Initialize the optional compressor to None; when pre-compression is enabled, the real compressor still replaces it. Topic segmentation can then initialize normally without pre-compression.

Test

uv sync --extra dev
uv run --project . python -m pytest -q tests/test_lightmemory_initialization.py tests/test_sensory_memory.py

Result: 3 passed.