CRITICAL: PNG compression completely broken - 1.4MB files, FFmpeg failures

[krystophny]

CRITICAL USER-BLOCKING ISSUE

PNG compression is completely broken causing:

• File sizes 1.4MB instead of ~24KB (60x bloat)
• FFmpeg animation failures: inflate returned error -3
• All PNG output unusable for animations

EVIDENCE:

• File sizes: show_output.png = 1,440,884 bytes (should be ~24KB)
• FFmpeg errors: [png @ ...] inflate returned error -3
• ZLIB decompression fails on all generated PNGs

USER IMPACT:

• Animations completely broken
• PNG files unusable in external tools
• Massive storage waste

ROOT CAUSE:
ZLIB compression implementation broken in custom PNG backend

REQUIRED FIX:
Fix ZLIB compression in src/external/fortplot_zlib* modules

This blocks ALL PNG functionality and animations.