Rename all DiagCode enum variants in the lexer to use the STRUCT_ prefix
to match the specification. This clarifies that these diagnostics relate
to structural/lexical issues in PDF documents.
Changes:
- InvalidName -> StructInvalidName
- InvalidHex -> StructInvalidHex
- InvalidOctal -> StructInvalidOctal
- InvalidStreamHeader -> StructInvalidStreamHeader
- UnexpectedEof -> StructUnexpectedEof
- UnterminatedString -> StructUnterminatedString
The hex string lexer implementation was already correct, with proper
handling of:
- Hex digit pair decoding
- Embedded whitespace (PDF spec 7.2.2)
- Odd-length zero padding: <4> -> \x40 (dangling nibble is HIGH)
- Invalid character diagnostics
- Unterminated string diagnostics
All 16 hex string tests pass, including critical tests for odd-length
padding and error handling.
See: notes/pdftract-2hm4.md
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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