Design vector sizing calculator UI

## Summary

Design review and sign-off for the vector sizing calculator before it ships in docs-content.

Implementation (for review): https://github.com/elastic/docs-builder/pull/2736

## Status

| Item | Status |
|------|--------|
| Interactive UI implemented (React + EUI) | Done in PR #2736 |
| UX polish (layout, typography, advanced section, formulas panel) | Done |
| Design review | Pending |
| Sign-off | Pending |

## Preview

- **PR preview (calculator):** https://docs-v3-preview.elastic.dev/elastic/docs-builder/pull/2736/syntax/vector-sizing-calculator
- **Local:** `dotnet run --project src/tooling/docs-builder -- serve` → `/syntax/vector-sizing-calculator`

## UI surfaces to review

- **Configuration panel** — vectors/dimensions inputs, element type, index type, quantization, replicas; collapsible advanced settings (HNSW, DiskBBQ, off-heap %)
- **Results panel** — per-replica disk/RAM hero, cluster totals, detail lines, disclaimer and doc link
- **Formulas panel** — collapsible disk / RAM / cluster formula breakdown
- **Responsive / docs chrome** — behavior inside Elastic Docs layout (not standalone app chrome)

## Scope

- Review against Elastic Docs design standards (spacing, typography, color, EUI usage, accessibility)
- Note required changes or approve as-is
- Hand off any follow-up implementation to Docs Eng (same PR or follow-up)

## Out of scope

- Directive / build plumbing (#3051)
- Formula accuracy (BBQ constant) — docs-content-internal#1031
- Embed placement in docs-content — docs-content-internal#1030

## Prerequisites for embed (#1030)

- [ ] Design sign-off recorded on this issue

## Acceptance criteria

- [ ] Configuration, results, and formulas panels reviewed
- [ ] Mobile / narrow viewport checked
- [ ] Sign-off comment added (approve or list of changes)
- [ ] Any blocking design changes tracked (issue comment or linked follow-up)

## Owner

@itsalexcm

Coordination: @KOTungseth