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

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

```css
@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).

Start by locating the configuration parser and the in-memory pipeline from issue #29, then inspect how presets and input files are currently represented. Implement support for the listed CSS directives, source glob resolution, the built-in Tailwind preset, and theme merging; done means a CSS entrypoint can drive the complete configuration flow.

Written by the indexing model from the issue text.