Fix clear_cache() malformed .cols/.rows paths

[drussellmrichie]

clear_cache() built the column/row fragment paths as f"{path}.cols{ext}" (missing the dot before the extension) for the existence check and f"{path}..cols.{ext}" (double dot) for removal — so the existence check never matched and the large .cols.parquet / .rows.parquet column-diff caches were silently left on disk. This uses the correct {path}.cols.{ext} / {path}.rows.{ext} templates.

Adds a regression test (tests/test_clear_cache_paths.py) that creates the full cache artifact set and asserts a clean sweep.

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[nikhilsi]

Independent confirmation: at 7952236 we reproduced the malformed .cols /
.rows path handling in clear_cache exactly as described (the existence
check missing the dot, the removal path doubling it), so the sibling files
were left behind after a clear. With this PR applied on top of 7952236,
clearing a cached entry removes the .cols and .rows siblings as
expected, and the included regression test passes. Works for us.