fix(ui): pull the Markdown heading ladder back to 1.5x the body

[zhangfand]

What this PR does

The rhythm retune in #402 opened the standard heading ladder to 16 → 20 → 24 → 30 so each level stepped by at least 1.2. Read in a chat reply, that ladder is too tall: an h1 at 1.9× the body and an h2 at 1.5× read as page titles, and a reply that opens with ## looks like a document rather than a turn.

The standard ladder now climbs 16 → 18 → 20 → 24 and tops out at 1.5× the body. Line boxes stay on the 4px grid: 24, 24, 28, 32. Weight stays at 600, and every heading keeps its space-before and space-after from #402, so the proximity rule still holds. Compact density is unchanged at 14 → 16 → 18 → 22, which is the same ladder shape one step down.

The third rule in the MARKDOWN TOKENS comment now says why the ladder stops where it does: prose here is a turn in a conversation, not a page with a title. The rules panel on /dev/markdown-rhythm says the same.

Design & Invariants

• A Markdown heading never exceeds 1.5× the body size at standard density. Hierarchy between levels comes from size and space; weight stays flat at 600.
• markdown-tokens.test.ts pins the new sizes and line heights. The proximity and edged-block ratios it asserts are untouched.

Test plan

• pnpm typecheck
• pnpm --filter @rome-os/ui test (675 passed) and pnpm --filter rome-web test (1487 passed)
• /dev/markdown-rhythm under pnpm start:web:mock: computed sizes read back as h2 20/28, h3 18/24, h4 16/24 at standard, compact unchanged
• A real long reply on /chat against a running backend

🤖 Generated with Claude Code

[zoolsher]

Code Review: ✅ APPROVE

This PR pulls the standard-density Markdown heading ladder back from 16 → 20 → 24 → 30 (topping out at ~1.9× body) to 16 → 18 → 20 → 24, capping h1 at exactly 1.5× the body so a chat reply reads as a conversational turn rather than a titled page. Three files change in lockstep: the --markdown-heading-* tokens in styles.css, the pinned expectations in markdown-tokens.test.ts, and the rules-panel copy in MarkdownRhythmPage.tsx.\n\nI verified the numbers end-to-end. New line boxes (32/28/24/24 for h1–h4) all land on the 4px grid, every referenced font-size/line-height token exists in the scale, weight stays at 600, and the space-before/space-after tokens are untouched so the proximity assertions (before ≥ 1.5×gap, before/after ≥ 2.5) still pass. Compact density is genuinely unchanged. The removed "steps by ≥1.2" claim was correctly dropped from both the comment and the dev-page copy since the new steps are smaller. A repo-wide grep found no stale references to the old ladder. Clean change, no findings.

Verdict: APPROVE — A self-consistent, well-tested design-token retune of the Markdown heading ladder with matching test and dev-page updates; no bugs or design issues found.

No issues found. Code looks good!

---

Automated review by RomeOS Code Review · commit 6e6e332