Next.js v16.2.0-canary.61: Instant nav fixes, leaner builds v16.2.0-canary.61

Squashed a nasty params leak in instant navigation dev mode and capped build workers at pages+1 to stop resource hogging. Also shipped the @next/playwright package with instant() testing helpers, because testing instant nav should actually be instant. Turbopack got some memory optimisations during compaction and a few other reliability fixes thrown in for good measure.