prevent widows, as the second feature package

The last two words of a paragraph or heading are joined by a non-breaking space,
so one word never falls alone onto its own line. Deferred from entry 11 for the
right reason: over rendered HTML this cannot tell prose from an escaped code span,
so it had to wait for a tree transform.

The interesting failure is worth keeping: written against goldmark alone it passed
six tests, and did nothing in the real engine. The typographer splits a text run
wherever it looks for a substitution, so a paragraph ending "hand." arrives as two
text nodes and the last of them holds no space at all. A version that inspects
only the last child therefore finds nothing to join. It now takes the whole
trailing run of text nodes, stopping at a line break or any markup, and there is a
regression test that builds both extensions together — the only configuration that
would have caught it.

The joined text becomes a String node, which carries its own bytes: a segment is
an offset into bytes every node shares, so editing the source in place is not
possible. That path still escapes, and a test says so, since otherwise this
transform would be an injection route.

PhaseMarkup is now empty in extensions.md, and honestly so: everything expected
there turned out to belong either earlier or later.

docs/extensions.md

@@ -65,7 +65,7 @@ wearing a disguise.
 |---|---|---|
 | `PhaseLoad` | raw bytes + frontmatter | translation fallback. *Not* includes: they turned out to be parse-phase, because splicing another file's parsed nodes into a page is invalid rather than merely awkward (ADR-0038) |
 | `PhaseParse` | the parsed Markdown tree | shortcodes, transclusion, image derivatives |
-| `PhaseMarkup` | rendered HTML fragments, code spans skipped | widows. Smart quotes and dashes turned out to be a Markdown parser option, and chrome localisation a template function (ADR-0034) — neither needed a phase |
+| `PhaseMarkup` | rendered HTML fragments, code spans skipped | nothing yet. Everything expected here turned out to belong earlier or later: smart quotes and dashes are a Markdown parser option, chrome localisation is a template function (ADR-0034), and widows are a tree transform — over rendered HTML none of them could tell prose from an escaped code span |
 | `PhasePage` | the assembled page object | OpenGraph, JSON-LD, related posts, series nav |
 | `PhaseOutput` | the final byte stream | minification, dithering, gemtext conversion |