Elide a long window title instead of cutting it mid-glyph

A title that did not fit was hard-cut at whatever character crossed the
button reservation. Nothing marked the cut, so a truncated name read as the
whole name - "annual-report-final-v7-reviewed-2026-with-appendix.ods -
LibreO" looks like a filename, not like a filename with its tail missing.

Titles now end in an ellipsis when they are shortened, which is what
Windows, GNOME and KDE all do, and it keeps the informative half: an
application or document title almost always begins distinctively and ends
in boilerplate. Whole characters are dropped until the ellipsis fits beside
what remains, so the result never overruns the buttons. A titlebar with no
room even for the ellipsis draws nothing, rather than a lone "..." that
says less than an empty titlebar does.

The mark itself is "…" where the system font has that glyph and "..." where
it does not. A missing glyph rasterizes to nothing at all, which would have
quietly reintroduced the invisible-truncation problem on any font without
it.

Also bounded the work: a client may set a title of any length - a browser
tab carrying a whole data URL - and every character cost a rasterization
before the layout could decide it did not fit. Measurement stops at 256
characters, well past anything legible in a titlebar, and a title that long
is elided many times over regardless.

Four tests: an over-long title fits its span and ends in the ellipsis mark;
a title that fits is left exactly alone; a titlebar too narrow for the
ellipsis draws nothing; a 20,000-character title never lays out more than
the cap. Checked on screen too, at 600px and at 220px.

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