explorer: collapse map stats panel by default

[rdhyee]

Summary

• Add a small Stats disclosure above the map stats grid.
• Keep the four stat boxes collapsed by default for novice users.
• Leave the existing stat value elements mounted so their values continue updating while hidden.

Closes #254.

Verification

• git diff --check
• /usr/local/bin/quarto render explorer.qmd
• python3 dev_server.py --dir docs --port 8078
• Headless Playwright smoke check confirmed the stats body is hidden on load, clicking the disclosure shows all four boxes, and the stats grid remains display: grid.

[rdhyee]

Superseded by #261, which shipped the consolidated runtime (stats-collapse #254 + chip #255 + facet-count fix #253) to isamples.org. Closing the PR. NOTE: the regression tests from these branches are not yet on upstream — they'll be re-added via one clean follow-up PR (incl. repackaging the count-race spec as a standalone .mjs to fix the [P2] default-suite//data dependency).