Function measureTextCells

Pinned, deterministic text measurement in terminal cell units.

• Unicode pins: 15.1.0
• Grapheme segmentation: UAX #29 core set
• Emoji width policy: wide (2 cells)
• Invalid UTF-16 surrogate sequences: replaced with U+FFFD
• Tabs are not treated specially (no expansion)

This function is cached for performance. Repeated measurements of the same text string return immediately from cache (O(1) vs O(n)).