Remove misleading LanceDB naming

[FANNG1]

Summary

• Rename the internal Lance scan operator and Python factory helpers away from LanceDB terminology.
• Update docstrings and examples to describe Lance datasets instead of LanceDB tables/client usage.
• Move tests from tests/io/lancedb to tests/io/lance and rename affected test files, classes, functions, imports, and explain-output assertions.

Closes #33.

Checks

• make check-format passed.
• make lint passed.
• uv run pytest tests/io/lance -v reported 278 passed, 5 skipped, 2 xfailed, 2 xpassed, but the process exited with code 139 after printing the summary.
• uv run pytest -p no:benchmark tests/io/lance/test_lance_count_pushdown_coverage.py tests/io/lance/test_lance_factory_function.py tests/io/lance/test_lance_point_lookup.py tests/io/lance/test_lance_reads.py -v reported 68 passed, 2 skipped, 2 xpassed, but also exited with code 139 after printing the summary.
• make typecheck failed on existing mypy issues across tests and a few existing Lance typing mismatches; no failures appeared specific to the naming change.

Notes

• The only remaining lancedb text from rg -i "lancedb|lance db" is the factual upstream URL https://github.com/lancedb/lance/....