Respect TeX control-sequence boundaries

[Happypig375]

Summary

• tokenize ordinary LaTeX control words as backslash plus ASCII letters only
• treat @ as a non-letter by default: \@ is a one-character control symbol, and \foo@bar ends at \foo
• keep apostrophe, relation, and star punctuation available to the normal parser
• consume delimiter whitespace only after control words, not control symbols
• reject backslash-prefixed command registrations that extend beyond their token boundary

Specification

• LaTeX2e control sequences
• \makeatletter and \makeatother

CSharpMath does not track catcode-changing contexts, so it uses the ordinary document boundary where @ has default catcode 12.

Regression coverage

• adjacent apostrophe, relation, and star punctuation
• @ terminating a control word and \@ acting as a control symbol
• control-word versus control-symbol whitespace
• acceptance of reachable command keys and rejection of unreachable keys
• parser atom types and exact serialization

Compatibility

Previously accepted custom command names containing @, such as \foo@bar, are now rejected as unreachable under the default tokenizer. Supporting package-style names would require explicit catcode-context support rather than an unconditional exception.

Validation

• full Core Release suite: 1,533/1,533 passed
• focused tokenizer/parser cases: 21/21 passed
• independent code review: approved
• git diff --check passed

Fixes #182

[Happypig375]

Resolved and merged as d8d5d004.

Clarification on @: it is not a CSharpMath custom extension, and the earlier Stack Overflow wording was only a quotation from that linked post, not the specification. Under the ordinary LaTeX document catcode regime, @ is a non-letter, so \@ is a one-character control symbol and \foo@bar ends the control word at \foo. Inside a \makeatletter context, LaTeX temporarily makes @ a letter, allowing package-style control words that contain it.

CSharpMath does not model mutable TeX catcodes or \makeatletter contexts. It therefore uses the ordinary document rule consistently: control words are backslash plus ASCII letters; control symbols are backslash plus one non-letter; delimiter whitespace is discarded only after a control word. Registrations that extend beyond that reachable token boundary are rejected.

Authoritative references: LaTeX2e control sequences and \makeatletter / \makeatother.

Full Core validation passed 1,533/1,533 tests before merge.