A preprocessor directive between a setext title and its underline hides the title

Split out of #16, which closed with the two-line title reading. is_next_line_section? calls reader.peek_lines 2, and a PreprocessorReader peek runs the directive machinery first, so a directive standing between a title and its underline is gone by the time the pair is compared:

Section Title
ifdef::asciidoctor[]
-------------
endif::[]

Asciidoctor reads a level-1 section Section Title; we read a paragraph, the two directives, and a listing block the ------------- opens. Our nextLine is the RAW line below, not the preprocessed one.

The fix is not just a preprocessor-aware peek. A formatter never resolves a conditional, so the directives have to survive into the output, and == Section Title followed by the directive pair puts ------------- at a block start where it opens a listing block again. Any repair has to decide what to emit for a title whose underline is conditional.

Corpus cases (quarantined on fidelity, tagged #16 in tests/conformance/quarantine.json):

• sections_test.rb#should preprocess second line of setext section title#0
• sections_test.rb#should preprocess second line of setext discrete heading#0

Ledger family gap:setext-title (scripts/block-structure-corpus.json), 2 entries.

Start with is_next_line_section? and reader.peek_lines 2, then trace how PreprocessorReader handles the directive before the setext title comparison. Review the two quarantined cases in tests/conformance/quarantine.json and the gap:setext-title entries in scripts/block-structure-corpus.json. Done means deciding and validating how a conditional underline and its directives are represented without creating a listing block.