Turn ligatures off with a Ligatures setting, in the shell that shapes text

The SDL shell shapes words with HarfBuzz, so a font's `->` and `!=` draw as
ligatures, and there was no way to have the plain glyphs back short of
changing font. `Ligatures` is a toggle, on by default; off, no cell goes to
the shaper and every cell draws its own glyph, exactly as a font without
ligatures does.

It exists only where it means something. A new `ligatures` capability, true
for the gui shell alone, gates it like Font and WindowOpacity are gated: it is
not a builtin elsewhere, has no leader path, and Config does not list it,
rather than print a row the TTY could never change. macOS draws CoreText
ligatures of its own, but nothing there reads the setting, so it stays off
there. The table check that kept every toggle unconditional now lets the
ligatures toggle, and only it, carry a capability, and requires that it
carry `ligatures`; every other setting's rule is as it was.

The gui keeps the setting beside its text caches, which were resolved under
it: when the core's value changes, the per-codepoint cells (which record
whether a cell is shaped) and the shaped words are dropped, and the frame the
toggle asked for draws every cell again. The atlas keeps its glyphs: plain
ones draw either way, and a ligature's strip is reused when it comes back.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

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