Support the \\not relation command

[Happypig375]

Summary

• support literal and control-word relation targets for \not
• reject targets that are not exactly one script-free relation with a useful diagnostic
• use an explicit table for every semantically exact registered negation instead of synthesizing \n... command names
• prefer Unicode-precomposed negated operators when available and use U+0338 COMBINING LONG SOLIDUS OVERLAY only when no precomposed form exists
• serialize registered, precomposed-unregistered, scripted, and repeated negations canonically and round-trip them

Semantics and specifications

LaTeX itself does not parse \not as a one-argument command: the kernel declares it as a \mathrel glyph, and defines \neq and \ne as \not=. See the official LaTeX2e kernel definitions of \neq and \ne and \not declaration. The LaTeX2e reference likewise describes \not as a long solidus that overstrikes the following relation.

CSharpMath deliberately differs by consuming one Relation. Within that parser model, rendering follows Unicode Technical Report #25: normalize to a precomposed negated operator whenever one exists, otherwise retain the base relation plus U+0338. Exact registered commands such as \neq, \nleqslant, and \nVdash remain canonical. Refinement relations such as \lneq, \lnapprox, and \subsetneq are not incorrectly inferred as simple negations.

Rendering coverage

Three distinct paths have committed display and inline PNG baselines at 32359727:

• registered named negation \not= → U+2260: display, inline
• Unicode-precomposed \not\approx → U+2249: display, inline
• genuine fallback \not\propto → U+221D plus U+0338: display, inline

Testing

• full Core suite: 1,583/1,583 passed
• exhaustive exact-mapping theory: 41/41 passed
• production glyph-layout structural checks passed
• SkiaSharp golden rendering suite: 348/348 passed with exact PNG comparisons
• independent code review: approved with no findings
• git diff --check passed

Builds on merged #276

Fixes #233

[Happypig375]

Resolved in 32359727, with the semantic divergence made explicit.

LaTeX itself declares \not as a relation glyph that overstrikes what follows; it is not a parser prefix that selects a named negation. The kernel even defines \neq and \ne as \not=. Sources: official LaTeX2e kernel definitions, official \not declaration, and the LaTeX2e \not reference.

CSharpMath intentionally keeps its one-Relation parser model, but now applies that deviation consistently:

• every semantically exact registered negation is selected through an explicit table, including inequalities, precedence, membership, turnstiles, triangles, subsets/supersets, congruence, parallel/mid, and the six arrow pairs
• command-name synthesis was removed, so refinement symbols such as \lneq, \lnapprox, and \subsetneq cannot be mistaken for simple negations
• all remaining relations are NFC-normalized after adding U+0338, following Unicode Technical Report #25: use a precomposed operator when one exists and a combining overlay otherwise
• serialization NFD-decomposes unregistered negations, so both precomposed and combining forms round-trip as \not <relation>

The committed PNG baselines exercise all three rendering paths:

• named \not=: display, inline
• precomposed \not\approx as U+2249: display, inline
• non-precomposable \not\propto as U+221D plus U+0338: display, inline

Validation passes 1,583/1,583 Core tests and 348/348 exact SkiaSharp golden comparisons; independent review found no issues.