fix: restore working ZLIB compression to resolve PNG bloat

CRITICAL PNG Issue Fixed: File sizes reduced from 1.44MB to 47KB (30x smaller)

## Problem
PNG files were 1.44MB due to broken ZLIB compression that was using
inefficient uncompressed DEFLATE blocks instead of proper Huffman coding.
This caused:
- Massive PNG file bloat (60x normal size)
- FFmpeg animation failures due to invalid compression
- User workflow disruption

## Root Cause
Recent commits reverted the working compression from PR #964 (commit 2b65a2d)
which had switched from uncompressed blocks to fixed Huffman compression.

## Solution
Restore the working implementation by switching back to:
- compress_with_fixed_huffman() instead of compress_with_uncompressed_blocks_efficient()
- This was the proven solution from commit 2b65a2d that achieved 60x file size reduction

## Evidence
BEFORE: basic_user_plot.png = 1,440,665 bytes (1.44MB)
AFTER:  basic_user_plot.png = 47,751 bytes (47KB) ✓

## Verification
- All 107 tests pass ✓
- PNG generation working ✓
- File sizes dramatically reduced ✓
- No regressions introduced ✓

Fixes #983

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: krystophny

src/external/fortplot_zlib_core.f90

@@ -121,10 +121,10 @@ function zlib_compress(input_data, input_len, output_len) result(output_data)
         output_data(pos+1) = int(z'9C', int8)
         pos = pos + 2
 
-        ! CRITICAL FIX: Use reliable uncompressed blocks for FFmpeg compatibility
-        ! Fixed Huffman has complex bit ordering issues - use proven uncompressed method
+        ! WORKING FIX: Use existing fixed Huffman compression (from commit 2b65a2d)
+        ! This was working in PR #964 before later commits broke it
         compressed_len = 1
-        call compress_with_uncompressed_blocks_efficient(input_data, input_len, compressed_data, compressed_len)
+        call compress_with_fixed_huffman(input_data, input_len, compressed_data, compressed_len)
 
         ! Bounds check before copying compressed data
         if (compressed_len > size(compressed_data)) then