perf(core): clone row template and slice active rows

[lukecotter]

Summary

Two small allocation reductions on core hot paths, both behaviour-preserving.

Changes

• Row.createElement clones a shared template element (cloneNode(false)) instead of running createElement + classList.add + setAttribute for every row.
• RowManager.setActiveRows copies with Array.slice() instead of Object.assign([], activeRows) (and drops a dead double self-assignment). Object.assign([], arr) falls onto V8's generic property-copy path and is dramatically slower than slice() on large arrays; this runs on every pipeline pass.

Benchmarks (Node, 250k rows, median of trials)

• Row.createElement 250k: +41% (1.7×)
• setActiveRows copy: +99% (≈166×) — removes a ~23ms hitch per pipeline pass on large datasets.

Tests

Full unit suite green. No behaviour change.