Harden spatial-media MP4 box parsing #1913

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Crafted MP4/MOV atoms could cause invalid Container casts, mismatched array deallocation, or copying unread heap memory to output. This hardens parsing and metadata injection against malformed box sizes and types.

• 
  

  Box ownership and copying

  
  
  • Use delete[] for array-backed box contents.
  
  
  • Write only bytes successfully read from input.
  
  
  
  


• 
  

  Bounds validation

  
  
  • Retain 64-bit extended box sizes through validation.
  
  
  • Use subtraction-based bounds checks and reject undersized headers/padding.
  
  
  
  


• 
  

  Container safety

  
  
  • Require constants::Container before traversing or modifying container contents.
  
  
  
  

if (iPos > iEnd || iSize < iHeaderSize

    || iSize > static_cast<uint64_t>(iEnd - iPos))

  return NULL;

• Fixes #1911