debt: renderVisibleRows rebuilds innerHTML on every scroll frame

[Kpa-clawbot]

Even with RAF throttling, renderVisibleRows sets tbody.innerHTML on every scroll, causing full DOM teardown+rebuild of the visible slice (~60 rows). The spacer elements created via createElement get orphaned and recreated each time. Minor perf concern — consider diffing visible range and only updating changed rows.

Ref: PR #402

[efiten]

Fix in PR #596.

Approach: incremental DOM diff instead of full `tbody` rebuild on each scroll frame.

• `buildFlatRowHtml` / `buildGroupRowHtml` now emit `data-entry-idx` on every `` (including expanded group children)
• `renderVisibleRows` computes overlap with the previous rendered range: if overlap exists, only the delta rows at the top/bottom edges are added or removed via `insertAdjacentHTML` / `querySelectorAll(...).forEach(r => r.remove())`
• Full rebuild retained for initial render and large scroll jumps past the buffer
• Spacer elements are reused rather than recreated each scroll frame

Result: on a normal scroll step the browser processes a handful of DOM mutations instead of ~60 teardowns + rebuilds.