🛡️ Sentinel: Fix SIGBUS on input/output path collision

[kassoulet]

This PR fixes a critical vulnerability where the application would crash with a SIGBUS signal if the user accidentally specified the same file for both input and output. This happened because the input file is memory-mapped for high-performance processing, and opening it again for writing (with truncation) invalidated the memory mapping.

The fix involves:

1. Moving the output path determination logic to the beginning of the main function.
2. Using std::fs::canonicalize to compare the absolute paths of the input and output files.
3. Returning a clear error message instead of proceeding if the paths are the same.

This change is under 50 lines and follows the security requirements for the project.

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PR created automatically by Jules for task 15379727575619323244 started by @kassoulet

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