[adarshsm]

Fixes #1245.

Popup currently calls setContent() for every HTML child. Each call replaces the previous content, so only the last child survives. This also removes the earlier chart containers when multiple Vega elements are added to a popup.

For multiple children, append their DOM nodes to one container and set the popup content once. Keep the existing single-child markup and child variables used by chart initialization. For lazy popups, load all children together in one click callback. Empty child output is ignored.

Added eager/lazy regression tests; both fail before the fix and pass afterward. An inverted multi-child condition also makes both tests fail.

Verification

• pytest tests/test_map.py -q: 23 passed, including the new eager/lazy cases and the existing single-child rendering checks.
• Local runtime check using real Leaflet 1.9.3 and jQuery 3.7.1 in jsdom: unmodified upstream retains only Second; the fix retains First and Second through loading and reopening in both modes, including an empty child. This checks popup DOM behavior, not Vega chart drawing or the Selenium suite.
• Broader non-browser suite: 397 passed, 4 failed. The failures are the same missing-geodatasets and Selenium-dependent PNG failures verified on unmodified upstream earlier in this run.
• Scoped ruff check and git diff --check: passed.

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