Perf‑by‑design runtime

Contracts that protect hot paths in viewport/virtualization.

1) Hot‑path invariants

• row pool reuse
• bounded buffers for visible rows
• stable callback signatures (no extra allocations)

2) Contract tests

• perfHotPath.contract.spec.ts
• gate: pnpm run quality:perf:datagrid

3) Budgets

Row models and interaction benchmarks must pass p95/p99 budgets.