Security: Fix unsafe pointer buffer overread in TextualStl parser

[trek-e]

Summary

• Fixes high severity out-of-bounds memory read vulnerability in TextualStl.cs
• The text-based STL parser used unsafe pointer operations without bounds checking
• Malformed STL files could cause reads past the end of the file buffer

Security Impact

• Severity: High
• Attack Vector: Maliciously crafted STL files with truncated data
• Impact: Information disclosure (reading uninitialized memory), potential crash

Changes

1. Added endOfFile pointer tracking throughout all parsing methods
2. Added bounds checks before every pointer dereference
3. Added maximum facet count limit (10 million) to prevent memory exhaustion
4. Changed parsing methods to return bool for graceful failure handling
5. Graceful handling of truncated/malformed files instead of crashes

Test plan

• Verify normal STL files still parse correctly
• Verify truncated files are handled gracefully without crashes
• Verify very large facet counts are rejected

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