Dataset page: machine-use block becomes an endnote

[neuromechanist]

Closes #300

Summary

• Demotes the "How to use the data (for agentic research)" disclosure (src/components/UseThisData.astro, rendered from src/pages/dataset/[id].astro) from a bordered/elevated card to a plain endnote under a hairline rule.
• Summary title drops from --fs-xl semibold to --fs-sm medium; the hint drops from --fs-sm to --fs-xs. Both are now below --fs-base, matching the body text the block already used.
• Container styling (background: var(--color-bg-elevated), border, border-radius) is replaced with a single border-top hairline rule, so the collapsed line reads as a footnote to the page rather than another content card competing with the README/BidsTree cards above it.
• No change to markup structure, content, section data, wording, copy affordance, or the markdown mirror (/dataset/<id>.md) — only CSS hierarchy/size. The block is still a native <details>/<summary>, still collapsed by default (no open attribute, no persisted state), and still sits last in the content column (unchanged from Collapse and demote the agent usage block (v0.2.6 regression) #297).
• JSON-LD / structured data on the page is untouched; this only touches the visible disclosure's presentation.

Why

Companion to nemarOrg/nemar-cli#1250 / website#300: #297 already fixed placement (collapsed, moved to the end of the column) but left the collapsed summary styled like a headline on an elevated card, so it still visually competed with human-facing content even though it no longer pushed anything down the page. This PR only changes hierarchy/size, not placement or content.

Testing

• bunx biome check src/components/UseThisData.astro test/use-this-data-placement.test.ts — clean
• bun run test — 72 files / 1693 tests pass, including 3 new + 5 existing placement/sizing guards in test/use-this-data-placement.test.ts
• Mutation-proved the new size guards: reverting the title to --fs-xl makes the new test fail as expected, confirming it isn't a false-positive pass
• bun run typecheck (astro check) — 0 errors (pre-existing unrelated hints only)
• NEMAR_SKIP_OG_GENERATE=1 bun run build — succeeds
• Visual check via a static reproduction of the component's markup + tokens.css loaded in headless Chromium: collapsed and expanded states in both light and dark theme, plus a 375px-wide viewport — endnote reads as subordinate to the README card, text remains legible, rows stack to one column, and commands stay selectable/copyable when expanded

Notes / open question

The component currently has no dedicated "Copy" button — commands are rendered as plain <code> text (still selectable/copy-pasteable by hand), unlike the button pattern used in ActionBar.astro/CiteExport.astro elsewhere on the page. Per the instruction to change hierarchy/placement/size only, I did not add new copy-button UI here. Flagging in case the intent for #300 was also to add a dedicated copy affordance to this block — happy to follow up in a separate PR if so.

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