delete the widows feature; line breaking is CSS

The human asked whether widow prevention belonged in the backend at all. It did
not, and it broke two rules already written down: the theme contract says the
engine decides nothing about how something looks, and ADR-0034 says authored body
text is the author's — while this inserted U+00A0 into that text.

The practical harm follows from the layer error rather than from a coding mistake.
The engine cannot see the line box, so joining the last two words is a guess that
can overflow a narrow viewport, and a reader copying the paragraph gets a
non-breaking space in their clipboard. `text-wrap: pretty` and `text-wrap: balance`
in the reference stylesheet know the line box and need no bytes in the content.

108 lines of engine deleted for one CSS declaration. The typographer stays: turning
`--` into an en dash is a text transformation no stylesheet can express, which is
exactly the distinction the new layer test draws.

Also worth recording: this took the Extensions counter from 3 back to 2. A threshold
reached by a feature that should not have existed was never a threshold.

internal/render/templates/theme.css

@@ -1,7 +1,9 @@
 /* Reference theme: legibility only, no design opinions (ADR-0026). */
 html { font-family: Georgia, serif; line-height: 1.6; color: #1a1a1a; background: #fdfdfb; }
 main { max-width: 34rem; margin: 3rem auto; padding: 0 1rem; }
-h1, h2, h3 { line-height: 1.25; font-weight: 600; }
+h1, h2, h3 { line-height: 1.25; font-weight: 600; text-wrap: balance; }
+/* Line breaking belongs to the browser, which knows the line box no engine-side guess can see (ADR-0045). */
+p, li, figcaption { text-wrap: pretty; }
 a { color: #1a4d7a; }
 img { max-width: 100%; height: auto; }
 pre, code { font-family: ui-monospace, monospace; font-size: 0.9em; }