An indented setext title under a substituting directive opens a literal block and loses the section

Found by the #327 review (verified at f22f1ded and at its base 5b510b4b: render-different under both programs at both, so not a regression of #327).

ifdef::x[body]
 Title
-----
more

With x undefined, Asciidoctor Ruby 2.0.26 and @asciidoctor/core 4.0.11 both read <h2 id="_title"> Title</h2> plus <p>more</p>. The formatter writes

ifdef::x[body]
 Title

----
more
----

which both programs read as a literal block plus a listing block. Same for Title over ------ and for the pair with no more line.

Mechanism. The indented line opens a literal delimitedBlock, not a paragraph, so BlockReading.openingLine (#309) is never recorded and neither half of #327's pair rule (underlinesTheTitleAbove, src/print/join.ts) can fire. The two-readings fact exists only on prose blocks.

Relation to #207. #207 is the no-directive case and loses only the leading space; this one loses the structure. Related to #327 (the unindented pair, fixed) and #328 (delimiter directly under the directive).

Closes when the indented first line under substituted content records its two readings the way a paragraph does (or the literal-paragraph reader declines the line where the deleted reading is a setext title) and the pair is written back; red-first pins for the three shapes above; render-equal under both programs; fixed point.

Start by tracing the literal delimitedBlock reader and the pair-handling logic in src/print/join.ts, focusing on why openingLine is not recorded for the indented line under substituted content. Add red-first coverage for the three shapes described, then verify render equality under Asciidoctor Ruby and @asciidoctor/core and confirm the formatter reaches a fixed point.