Library API: composable `matched_tokens()` + `generate_from_tokens()`

[goulvenclech]

Expose two composable library functions for framework integration:

• matched_tokens(sources, engine) → Vec<&str> — extract tokens from source content, filter to those that resolve against the engine, deduplicate, sort. No CSS generated.
• generate_from_tokens(tokens, engine) → String — take a token list and produce the final CSS string.

Frameworks call them sequentially and can stop after the first to hash the token list for cache invalidation, without paying the cost of CSS formatting.

Depends on #30.