Stream CSS tokens and resume unfinished input

[adamziel]

Lets a CSS token span input chunks and continue in a new PHP process without restarting the file.

Review order: #315 (merged) → #316 → #317 → #314.

The processor keeps unfinished input and parses it again after another read. Whole strings and streamed input use the same next_token(), getters, and value setter. flush_processed_css() returns edited, completed input and releases its source bytes. The cursor contains the remaining bytes and whether more input is expected; the caller saves it with the source and output offsets after writing the output.

This is layer 3 of the CSS stack, based on #316. It adds input, flushing, and the token cursor together. It contains no URL mappings or rewrite_chunk(); those remain in #314. ASCII spans are scanned together so reparsing long identifiers and URLs does not walk every byte in PHP.

A large token increases memory use, cursor size, and repeated parsing work. Token size is not capped. The README spells out that cost and links the real-file caller.

Testing

The streamed tokens match the whole-string corpus, including one-byte reads, escapes, and invalid UTF-8. Separate PHP processes edit a multi-chunk file, stop before or after a saved checkpoint, and resume with the unfinished token. The written file must match whole-string edits without missing or duplicated bytes. The full DataLiberation run completed with 2,618 tests and 16 skips. PHPCS passes.