1.0.4: Let a plugin ship a stylesheet the panel can wear

A theme is a manifest block: a name, a stylesheet inside the plugin, and the
panels it applies to. The panel loads its own stylesheet first and this one
after it, so a theme is a file of token overrides rather than a rebuild of
everything, and docs/themes.md says which tokens are worth starting from.

The path is checked like a path, because this is the one thing a plugin ships
that reaches a browser as the plugin wrote it: relative, inside the plugin,
and a .css file. The panel serves it as text/css and will serve nothing else.

A plugin that ships only a theme now needs no runtime block and no entrypoint.
It is not a program: there is no listener for the panel to call, no secret to
hold, and an entrypoint would be a PHP file that never runs. Anything the
panel does have to call still has to declare one, which is what the rest of
that rule says.