Fix useLiveQuery loading status in on-demand sync

[KyleAMathews]

For live queries using on-demand sync mode, the collection was being marked as ready immediately when source collections were ready, even though the initial loadSubset hadn't completed yet. This meant isLoading was false while data was still being fetched.

This fix ensures that:

1. Live queries with on-demand sources track the initial loadSubset promise and show isLoading: true until it completes
2. The collection status remains loading until the first data load finishes
3. Subsequent loadSubset calls (pagination/windowing) do NOT affect the ready status - only the first load matters

Changes:

• Add hasMarkedReady and hasSetupLoadingListener flags to track initial ready state in CollectionConfigBuilder
• Modify updateLiveQueryStatus() to wait for first loadSubset to complete before calling markReady()
• Update subscribeToMatchingChanges() in CollectionSubscriber to track the initial loadSubset promise for on-demand sources
• Add comprehensive tests for the new behavior

🎯 Changes

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).