Support the rising diagonal ellipsis command

[Happypig375]

Summary

• register LaTeX \udots as the rising diagonal ellipsis U+22F0
• preserve it as an ordinary atom in scripts and array cells
• add parser, canonical serialization, and structural coverage

Fixes #301.

Validation

• CSharpMath.Core.Tests: 1,517 passed
• Debug Core build: 0 warnings, 0 errors
• Debug Rendering test build: 0 errors (one existing warning)
• Rendering test assembly including TestCommandDisplay.CommandsAreDisplayable: 538 passed
• bundled Latin Modern Math cmap verified to contain U+22F0
• format, diff, public API, and generated-artifact checks clean

Compatibility and maintenance cost

This adds one entry to the existing command table and uses the bundled font's existing glyph. It introduces no public API or fallback path.

Review

Independent senior review: approved with no findings.

[Happypig375]

Rebased onto the current master after #276 merged. The conflict resolution preserves all #276 control-sequence boundary tests and both #301 \udots tests.

Post-rebase validation:

• LaTeXParserTest: 293 passed
• parser + dictionary review scope: 314 passed
• independent conflict-resolution review: approved
• fresh Windows, Ubuntu, macOS, benchmark, and repository-format checks: all green