Add Quick Start lead with Docker prereq and spec-pick hint

[PunchTheDev]

Summary

Explorer Quick Start block was ○ ○ ○ on the BACKLOG — no lead, no prereq, no connection from step 3's --spec <id> substitution to "pick a spec on the left first."

Changes

• Playground.tsx L99–116: one lead <p> between the Quick Start head and the 4-command block: "Four commands from a clean machine to a scored eval result. Requires Docker. Pick a problem on the left first so step 3 fills in --spec automatically."
• Requires Docker carries a 201-char dotted-underline tooltip explaining what step 3 sandboxes + Docker Desktop / docker-engine install hints
• --spec in the lead is rendered in text-forge-accent <code> so the visual link to step 3's --spec token is obvious
• BACKLOG flipped ○ ○ ○ → ● ● ●

Verify

Puppeteer 1440×900 on /explorer:

• lead present, exact text match
• tooltip 201 chars on Requires Docker
• 0 new console errors (one pre-existing 404 unrelated)