Samples table and phase-msg 'Samples in View' diverge during point-mode cache-hit pans

[rdhyee]

Scoped out of #219. Documented in EXPLORER_STATE.md §6 table v2 viewport-coupling paragraph.

Bug

The two surfaces that report sample counts in the current viewport use different cache strategies:

• Phase-msg "Samples in View" / #sSamples stat box: written by loadViewportSamples() in zoomWatcher. Reuses cachedTotalCount from the previous query whenever the new viewport is within the previously-fetched padded bounds (isWithinCache(bounds) short-circuit). Designed to make panning feel smooth.
• Samples table (post PR Samples table: scope to current viewport bbox #219): re-queries on every viewer.camera.moveEnd. Always reflects the current 30%-padded viewport bbox.

Result: on cache-hit pans the table refetches and gets a slightly different total than what the phase-msg shows from its earlier fetch. E.g. table=147, phase-msg=153 from the prior fetch.

Primary #219 bug (table=6M while phase-msg=153 at the same viewport) is fully resolved; this is a smaller residual divergence.

Options

1. Make the table reuse the point-mode cache. When loadViewportSamples() short-circuits on a cache hit, signal the table to also short-circuit (don't refetch; trust the previous totals). Couples tableView to a zoomWatcher internal.
2. Make point-mode always refetch the count. Drop the cache-hit short-circuit for cachedTotalCount so each moveEnd refetches. Slight perf regression on rapid panning.
3. Make both share a single bbox-snapshot + count cache. Cleanest architecturally; more refactor.

How to reproduce

Zoom into point mode somewhere with >5K samples (e.g. Cyprus / Polis). Pan a small amount (within ~30% of the original bbox). Compare table row total to phase-msg "samples in view" — they should differ slightly.

References

• Cache short-circuit: loadViewportSamples() isWithinCache(bounds) branch in zoomWatcher
• Table refetch trigger: viewer.camera.moveEnd listener in tableView (PR Samples table: scope to current viewport bbox #219)