Fix critical security and data integrity issues in BLM parser

[pitbot]

CRITICAL FIX:

• Fix data leakage bug where $row_arr was not reset between iterations, causing fields from previous rows to bleed into subsequent rows when fields were missing (phpblm.php:130)

Security Improvements:

• Add file path validation to prevent arbitrary file reading
• Add file size limit (50MB) to prevent memory exhaustion attacks
• Validate file existence and readability before processing

Error Handling:

• Add comprehensive bounds checking for array access in getData() and getHeader()
• Validate BLM format has required sections (minimum 7)
• Validate required headers (EOF, EOR) exist before use
• Handle file_get_contents() failure
• Add bounds checking in data parsing loop

Code Quality:

• Improve variable naming (dataCount vs datac, rowLength)
• Add comprehensive documentation explaining BLM format structure
• Add meaningful error messages with context
• Use explode() limit parameter to handle edge cases with delimiters in values

All changes maintain backward compatibility while improving reliability and security.

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