Implement the remaining LaTeX command backlog

[Happypig375]

This epic tracks LaTeX commands that require distinct parser, model, or layout work. Native sub-issues are the source of truth; each has its own semantics and acceptance tests.

Implemented on the current pull-request queue:

• Support \overleftarrow and \overrightarrow #185 — \overleftarrow and \overrightarrow in Port iosMath 2026 commits to CSharpMath #264.
• Support the \not negation command #233 — \not in Support the \\not relation command #280 after its parser-boundary dependency Parse punctuation after LaTeX control words (\sin', \mu=, \Delta=) #182/Respect TeX control-sequence boundaries #276.
• Support the rising diagonal \udots ellipsis #301 — rising diagonal \udots in Support the rising diagonal ellipsis command #308.
• \dfrac, \stackrel, \underset, \colorbox, extensible macros, \idotsint, thin spaces, and amsmath modulo forms are already implemented or included in Port iosMath 2026 commits to CSharpMath #264.

Remaining native sub-issues:

• Support text small-caps and slanted font shapes #300 — \textsc and \textsl, split into the composable style-model and glyph-selection prerequisites Represent composable font family, weight, posture, and capitals #306 and Select slanted and small-cap text glyphs semantically #307.
• Lay out integer long division with \longdiv #302 — validated integer arithmetic plus a dedicated \longdiv multi-row display.
• Support scalable \middle delimiters #303 — context-aware \middle delimiters sized with their enclosing \left...\right group.
• Support the \lbrack and \rbrack square-delimiter aliases #309 — the \lbrack and \rbrack square-delimiter aliases.
• Support \mathrel and \joinrel for composite relations #310 — \mathrel atom classification and the \joinrel negative-math-space primitive.
• Support standard LaTeX relative size declarations #311 — standard LaTeX relative size declarations.

Completion criteria:

• Every child command accepts representative valid syntax and reports malformed arguments at the correct source position.
• The atom/display model preserves command semantics rather than using a merely visual substitution where sizing, spacing, or font shape differs.
• Canonical serialization is stable under parse → serialize → parse.
• Layout-affecting commands have structural measurement tests and focused cross-frontend rendering coverage.
• Close this epic only after the queued PRs merge and all native children are completed or explicitly closed as not planned.

Contributors should work on one child issue per focused pull request; the old mixed checklist is retained in issue history.

[Happypig375]

Blocked on #143.

[Happypig375]

Triage update: current master still rejects every unchecked command. I added \dfrac and \udots from #233, linked the separately scoped over-arrow and \not issues, and removed good first issue because this checklist mixes moderate and very hard layout work.