`.css` file as configuration entrypoint (Tailwind CSS v4 style)

[goulvenclech]

For ecosystem compatibility, the config entrypoint should be a .css file with custom at-rules:

@import "tailwindcss";
@source "../src/**/*.html";
@theme { --color-primary: #3490dc; }

This requires:

• Parsing custom CSS directives (@import, @source, @theme, etc.).
• Resolving @source globs to discover input files.
• Mapping @import "tailwindcss" to the built-in preset.
• Merging @theme values into the config.

Depends on #29 (in-memory pipeline).