Release partial mmaps when MemoryMappedDataset init fails

[mmshad]

Summary

• Wrap MemoryMappedDataset.__init__'s file-open loop in try/except that calls _close_mmaps() on failure and re-raises, so partial state does not leak through the exception traceback (pytest frames, logger.exception, post-mortem debuggers).
• Add close() for explicit release and __del__ for GC fallback.
• _close_mmaps suppresses BufferError (live views into the mapping) and ValueError (already closed by another path) so cleanup is always idempotent.

Closes #55

Test plan

• uv run pytest tests/unit/test_dataset_mmap_cleanup.py -v (4 tests).
• Full unit suite clean.

[Naeemkh]

Relying on del for cleanup is generally discouraged (interpreter shutdown ordering, reference cycles, etc.). The contextlib.suppress(Exception) is the right defensive move, but consider documenting that close() is the preferred path and __del__ is a safety net only.

[mmshad]

Relying on del for cleanup is generally discouraged (interpreter shutdown ordering, reference cycles, etc.). The contextlib.suppress(Exception) is the right defensive move, but consider documenting that close() is the preferred path and __del__ is a safety net only.

Good catch! Pushing a follow-up on this branch: docstring on __del__ flagging it as a GC safety net, and a tighter close() docstring naming the explicit path as preferred. Happy to also add __enter__/__exit__ for a context-manager path if you want it. Let me know.