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## Stack Position (2/7) in the router instrumentation stack. 1. #94755: router instrumentation: refactor client hook dispatch (1/7) 2. **#94766: router instrumentation: add transition start context (2/7)** 3. #94756: router instrumentation: add transition commit events (3/7) 4. #94765: router instrumentation: add commit route and prefetch context (4/7) 5. #94757: router instrumentation: add transition abort events (5/7) 6. #94758: router instrumentation: add route mismatch events (6/7) 7. #94737: router instrumentation: add transition settlement events (7/7) Parent: #94755 Next: #94756 ## What This PR Does Adds an experimental third argument to the existing start hook: ```ts export function onRouterTransitionStart(url, navigationType, event) {} ``` The event contains: - `id` and framework-captured Unix `timestamp` in milliseconds - `fromRoutes`: route patterns visible when navigation is dispatched - `prefetchIntent`: `full`, `auto`, or `none` The primary `children` route is first, followed by visible parallel slots in deterministic order. ## Design Start owns the context that is authoritative before navigation work begins: the screen being left and the caller's prefetch intent. Destination routes and the cache result actually used are reported by commit instead. Consumers correlate them by transition ID. Without `experimental.instrumentationClientRouterTransitionEvents`, the existing two-argument hook behavior is unchanged. ## Not In This PR - Destination routes or observed prefetch result - Commit, abort, mismatch, or settled hooks ## Reviewer Focus - Route pattern extraction, including dynamic and parallel routes - Primary route ordering - Link intent propagation and experimental gating ## Validation - `pnpm build-all` - `pnpm --filter=next build` - App Router instrumentation client E2E: 15/15 <!-- NEXT_JS_LLM_PR --> --------- Co-authored-by: Jude Gao <[email protected]>