test(ui): re-pin the transcript rhythm contract in one file

[Astro-Han]

Summary

packages/ui/src/styles.css told readers that the "compact == transcript" assumption was held by packages/ui/src/__tests__/markdown-rhythm-dom-contract.test.tsx. That file was deleted in #2462 and its stylesheet-side sibling (apps/desktop/src/main/__tests__/markdown-rhythm-contract.test.ts) in #2425, so the comment has been citing a guard that does not exist — the worst state to leave an assumption in, because the next person reads the citation and stops looking.

This restores the two assertions the comment is actually about, plus the ladder order #2348 exists to protect, in one file. Split across two they each stayed green against the half they could not see, which is how the stylesheet half got deleted without anything noticing.

Pinned:

• the ladder is declared on the --space-* scale in strictly increasing order — retuning a rung is a design call and stays green; list gaps meeting block gaps fails
• every rung declared is actually spent by a rule, so the ladder cannot become decoration while literals do the spacing
• the runtime hooks the selectors need are really emitted by MarkdownBody (data-maka-contract, data-density, astryx-markdown-heading, data-level, astryx-list-item). These names have a single upstream owner in Astryx and appear in Maka only inside selectors, so a rename kills every rule at once with nothing failing
• compact markdown still has exactly one caller — the bet the comment names: the table carries heading TYPOGRAPHY on a density key, which Astryx's own RFC (RFC: Density System — Theme-level + Nestable Density Provider facebook/astryx#839) says density must not do

Deliberately not pinned, because they are how the table is written rather than what it promises: the adjacent-sibling gap form, the hr rung, the ListItem padding reset, and the two heading size tiers. The comment now says so, so the next reader does not have to open the test to find out.

Why this is not what #2425 / #2462 retired

Those PRs removed markup/copy pins, duplicate presentation assertions, and CSS-structure suites that asserted how a rule was written. Nothing here asserts an appearance or a rule's shape: three of the four assertions are relations that must survive any retune, and the fourth renders the real component and checks the join between Astryx's output and Maka's selectors.

The stylesheet-reading half does not revive the deleted css-test-helpers module or the renderer-wide CSS aggregate it read — it reads packages/ui/src/styles.css directly, in the package that owns it.

Refs #2348, #2425, #2462

Verification

Seven mutations, each verified to fail the assertion that describes it and pass again on restore:

mutation	caught by
invert the ladder (--md-gap-list: var(--space-4))	ladder order
take a rung off the grid (--md-gap-block: 8px)	ladder order
hardcode a rung's usage (margin-block-start: 16px)	rung is spent
hardcode it while naming the rung in a comment	rung is spent (comments are stripped first)
rename data-maka-contract="markdown"	DOM hooks
cut density off before it reaches Astryx	DOM hooks
add a second density="compact" caller	transcript-only surface

Also run: npm --workspace @maka/ui run build + the new file under node --test (4/4 pass), npm --workspace @maka/ui run typecheck, npm run lint, npm run format:check. Not run: the full repository suite, left to CI.

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Claude Code — read the deleted suites out of git history, chose which assertions to restore and which to drop, wrote the test and the comment correction, and ran the mutation sweep above. The scope call (restore the invariants, leave the rule-shape assertions out) was reviewed by the human contributor.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No

[Astro-Han]

Folded into #3436 — the retune and the guard that makes it reviewable ship as one intent, per review feedback. Branch deleted; the commit is preserved verbatim there.