harden filename construction against integer overflow and OOB access

[metsw24-max]

This patch hardens filename construction logic in fileio.c against integer overflow, out-of-bounds access, and unsafe string operations.

Added precomputed length handling and overflow-checked capacity calculation

Fixed separator logic when output directory is empty to avoid out-of-bounds indexing

Added explicit destination-capacity overflow guards in compressed-name generation

Replaced unsafe suffix write with length-aware copy