Add native \uparrow and \downarrow symbols to TXF3

[DoctorDan1986]

Summary

• Add native \uparrow (U+2191, ↑ / NAND Sheffer stroke) and \downarrow (U+2193, ↓ / NOR Peirce arrow) to the two Latin Modern TXF3 font AppVars at slots 0xC5 and 0xC6, preserving existing slots through 0xC4 (\supset).
• Extend the reproducible font-generation symbol list, public font macros, and lexicographically ordered command lookup table.
• Add host symbol lookup assertions for both commands.

Fonts

• TeXFonts.8xv: 8,851 bytes (previously 8,795); 8×14 ink pixels per new arrow.
• TeXScrpt.8xv: 5,933 bytes (previously 5,907); 5×10 ink pixels per new arrow.
• New arrows are rasterized from the existing, unmodified Latin Modern Math OpenType source, using the same quantization, metrics, and MATH attachment calculations as tools/process_fonts.py.
• Rebuilt font containers were validated against the exact OpenType raster for the pre-existing \supset glyph; original glyph metrics and packed pixels remain unchanged.

Validation

• Generated font AppVar lengths, checksums, glyph offsets, metrics, and packed bitmap sizes verified locally.
• Symbol map checked for lexicographical sort order.
• Host C/CI suite to run on this PR; no claims of CEdev/physical-calculator validation.

The XNOR \odot request is deliberately out of scope.