Rename algorithms/map/longest_common_subsequence.py -> longest_common_substring.py

[keon]

Summary

• The file under `algorithms/map/` is misnamed: it actually implements longest common substring (contiguous), not subsequence. The module docstring, the function name (`max_common_sub_string`), and the Wikipedia reference URL inside the file all already say "substring" — only the filename was wrong.
• The true longest common subsequence already lives in `algorithms/dynamic_programming/longest_common_subsequence.py`.
• Updates the single import site in `algorithms/map/init.py`. The exported symbol name (`max_common_sub_string`) is unchanged, so no test or downstream caller needs to change.

Closes #2747.

Test plan

• `pytest tests/test_map.py` → 25 passed
• `from algorithms.map import max_common_sub_string` resolves and `max_common_sub_string("abcdef", "acdbef")` returns `"cd"`

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