A reflow join flips the reading of a trailing title or indented line (render-equal, parse-side divergence)

Found by the #58 reading-invariant design prototype: 6 depth-5 sweep shapes (3x a trailing .T flips from title to text, 3x a trailing indented line flips from literal-paragraph start to paragraph text) where joining two paragraph lines changes our own classifier's reading of the LINE AFTER the join, while the emitted bytes of that line are identical and the oracle renders both sides the same. Example: '* a\n[role]\npara\npara\n[[anc]]\n lit\n' formats to '* a\n[role]\npara para\n[[anc]]\n lit\n'. Render-equality implies our parse of one side diverges from the oracle, so each shape is simultaneously a latent reflow hazard and parse-side divergence evidence; the fix likely rides the #55/#57-family parse work. Mechanism M2 in the reading-invariant ledger (#58).

Start with the #58 reading-invariant design prototype and the #55/#57-family parse work referenced here; inspect the six depth-5 sweep shapes and Mechanism M2 in #58. Reproduce the supplied example and verify that joining lines no longer changes the classifier's reading of the following identical-rendering line.