Report mixed DoenetML versions to the containing page

[dqnykamp]

Implements #39.

Each document in an assignment carries its own DoenetML version, and every embedded viewer downloads and parses the multi-MB standalone bundle for its document's version — an assignment mixing versions multiplies that cost by the number of distinct versions on the page.

Two deliberate non-choices, per discussion on the issue:

• No automatic normalization — rendering old documents with a newer DoenetML version could change their behavior; that stays the consumer's explicit choice (normalize version in the source; saved student state survives, as its hash deliberately ignores version).
• Not just a console warning — invisible to the site's visitors. The condition is reported to the containing page through a new typed callback prop, and the page decides how (and whether) to display it:

<ActivityViewer
    source={source}
    reportWarningsCallback={(warnings) => {
        // warnings: ActivityViewerWarning[] — first member:
        // { type: "mixedDoenetmlVersions", versions: string[] }
    }}
/>

Reported once per distinct source analysis — not per render, not on callback-identity churn, and not repeatedly even when the host passes a fresh (new-but-equal) source object each render (deduped on the serialized warning content, the same sameness pattern propSetKey already relies on). A console.warn is still emitted for developers, and the README documents the cost and the normalization path.

Tests: vitest for the version collector (single doc / uniform collapse / mixed nested select in first-appearance order / empty container yields none); cypress component tests asserting the callback fires exactly once with the version list for a mixed assignment, still only once when the host re-renders with a fresh source object each render, and stays silent for a uniform one.

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