Add \uparrow, \downarrow and \odot to TXF3 math symbols

[DoctorDan1986]

Summary

Add three native Latin Modern TXF3 math glyphs for Boolean NAND, NOR, and XNOR:

Command	Unicode	Meaning	TXF3 slot
\uparrow	U+2191 ↑	NAND / Sheffer operator	0xC5
\downarrow	U+2193 ↓	NOR / Peirce arrow	0xC6
\odot	U+2299 ⊙	XNOR / circled dot	0xC7

This builds on the merged \supset addition (PR #1), preserving all existing glyph assignments.

• Register all three commands in tex_symbols.c and include/texfont.h.
• Classify them as binary operators for contextual TeX math spacing.
• Extend the Latin Modern font generator and regenerate both TeXFonts.8xv and TeXScrpt.8xv (including offset adjustments).
• Add symbol lookup and normal/script math-spacing regression cases, and update the supported-command reference.

Font validation (local)

• Input fonts match the repository's original Latin Modern OpenType source Git blob hashes.
• Existing \supset glyph reproduced byte-for-byte with the same rasterization/metrics process.
• Regenerated main: 8,887 bytes, SHA-256 b599f33a4231ad3c4983b05f8ed61662e81211d792460659d59522ec57f068ad.
• Regenerated script: 5,949 bytes, SHA-256 e32e7cddcf213f5f4ca6c2799444874705fd4ea8839ec145158e3a53b8185826.
• Both binaries passed TI AppVar checksum, TXF3 record ordering, bitmap bounds, two-bit padding, and complete payload continuity checks.
• New fonts' Git blob SHAs independently match the regenerated local binaries.

Native compiler/CI tests have not yet been run locally; this PR should be validated by upstream CI.

No GoNDR application or installer code is included in this change.

[Sightem]

thanks!