v0.1.8

Declare the dependency the keyframes were hiding

The four bespoke keyframes came back in 0.1.7 to fix overlays that
stayed mounted after closing. They were the wrong answer, and the
reasoning is already written at the top of lib/motion.ts: they were
referenced through Tailwind ARBITRARY values, which Tailwind compiles
unconditionally, so animation was set while the keyframes behind it
might exist nowhere -- the browser then never fires animationend. A
theme-registered utility is simply not emitted instead, which is why
0.1.6 moved to tw-animate-css and dropped them.

Measured rather than argued: nothing in src/ or dist/ references
nebula-fade-* or nebula-zoom-* as code -- the one hit in the published
dist/lib/motion.js is the doc comment saying nebula USED to. And
presence.js gained armDeadline in 0.1.6 (3 occurrences; none in
0.1.5), so a surface whose animationend never arrives is now removed
on a deadline rather than left in the document.

What is real is the dependency underneath. tw-animate-css was a
devDependency, inlined only into the compiled nebula.css, so every
consumer on the tailwind or unocss adapter had to supply it without
being told -- and the README said the opposite, listing the keyframes
as its replacement. It is now an optional peer, the README says what
to import, and the section explains why the old arrangement failed
badly rather than cosmetically.

Put back the keyframes every overlay closes with

An overlay leaves the DOM when its exit animation ends — the component
waits for animationend. Without a @keyframes to match the
animation: nebula-fade-out … the utility emits, the animation never
starts, the event never fires, and a closed dialog stays on the page as a
full-screen backdrop nobody can see and every click hits. The application
appears to stop responding, with nothing in the console.

These were in this file through 0.1.5 and went missing in 0.1.6, while the
README kept listing them as what this file contributes in place of
tw-animate-css. Consumers on the css adapter never noticed: nebula.css
is compiled here and carries its own copy. Everyone on tailwind or
unocss lost their overlays on upgrade.

Restored exactly as 0.1.5 had them rather than rewritten from the names.

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Changes since v0.1.7.