Rewrite instant validation to use depth-based URL boundary discovery (#90905)
Instead of pre-planning validation tasks by walking the tree and
accumulating `navigationParents`, the new implementation drives
validation by URL depth. Starting from the deepest URL segment and
working backwards, it builds a combined payload at each depth where the
boundary between shared (already-mounted) and new (being-navigated-to)
content is determined by counting URL-consuming segments. If the new
subtree at that depth contains any `unstable_instant` configs, the
payload is validated. If not, it moves to the next shallower depth.
This approach has several advantages over the task-based model:
- Route group layouts no longer create spurious navigation boundaries —
they don\`t consume URL depth, so they naturally share the boundary of
their parent URL segment
- The `requiresInstantUI` requirement propagates correctly through the
tree: a local `false` config opts the subtree out, a non-false config
opts it in, and no config defers to the OR-union of children slots
- Both the static-stage and runtime-retry payloads are built in a single
tree walk, avoiding the separate retry pass
- The `<head>` stage correctly uses Runtime when any segment in the new
tree uses runtime prefetch
next.js
60a847b0 - Rewrite instant validation to use depth-based URL boundary discovery (#90905)
