Fix TTY table emoji width for ZWJ and skin-tone sequences

[ewilderj]

A small fix to ensure tty-only users also get nice emoji alignment :-)

Modern terminals (Ghostty, WezTerm, Kitty, iTerm2) shape ZWJ sequences and skin-tone modifiers into single 2-cell glyphs, but string-width sums their components — so tables with complex emoji misalign in TTY mode.

This routes the TTY width and padding paths through a new markdown-overlays--string-width that splits into grapheme clusters and forces width 2 for ZWJ, VS-16, and Fitzpatrick modifiers.

Terminal detection is a small TERM_PROGRAM allowlist (Ghostty, WezTerm, Kitty, iTerm2), memoized on a frame parameter. Setting the markdown-overlays-modern-tty frame parameter to yes or no overrides the detection.

Tests cover the grapheme-width rules and the allowlist/denylist behavior.

[xenodium]

Thanks for the PR!

[ewilderj]

Thanks! FWIW the same TTY shaping fix would apply to the new renderer in xenodium/agent-shell#597 — its --table-needs-pixel-p covers ZWJ and regional-indicator pairs but not VS-16 or skin-tone modifiers, and it has no TTY path