fix(as-S4): reading measure — prose at 72ch, headings step by ~1.2

From the anti-slop run (design-dev, 2026-09-28). A rendered doc ran 110-120
characters a line and its h3 sat at 1.08x its body. Prose blocks in
.markdown-body are capped at 72ch (pre and tables keep the full width, where
they scroll), and h3/h2/h1 step at 1.2/1.44/1.73em. Measured in a real
browser: a long paragraph now reads under 76 characters across, and every
heading step is >= 1.18.

Contract: as_antislop S4. Falsifiers: antislop.toml 43/43 proved (S1-S4);
all 12 tables 323/323 proved on this tree.

docs/contracts/as_antislop.contract.md

@@ -109,9 +109,17 @@ The run found that the Booth is sound on desktop and has a set of problems a vie
 - `.vname` already ellipsises; the detector measures the clipped inner width.
 - The filmstrip clips its next frame at the edge on purpose: the clipped frame is the "there is more" cue of a horizontal scroller.
 
-## S4 to S6
+## S4 — reading measure
+
+**The rule:** a rendered document reads at a book's measure and says its structure with size.
+
+- **Measure.** Prose blocks in `.markdown-body` (paragraphs, lists, block quotes, headings, definition lists) are at most `72ch` wide. Wide blocks (`pre`, tables) keep the full width, where they scroll.
+  - *Falsifiable:* on the doc view at 1280 wide, a long paragraph measures at most 76 characters of its own font across: `test_doc_prose_reads_at_a_book_measure`.
+- **Heading scale.** h3 : body, h2 : h3 and h1 : h2 are each at least 1.18 (h3 `1.2em`, h2 `1.44em`, h1 `1.73em`). Before this, h3 was `1.08em` over its body.
+  - *Falsifiable:* `test_doc_headings_step_by_size`.
+
+## S5 and S6
 
 These are added as each slice is staged:
-- S4, reading measure;
 - S5, interaction and screen readers;
 - S6, the operator's rulings (tagline, needs-you stripe, matte brand dot, the Wipe-now stripe on `::before`).