### What?

When instant validation flags a route that can't load instantly, the
overlay shows a warning. This PR (1) distinguishes nav-phase warnings
from initial-render ones in the overlay text, and (2) clears the
nav-phase warnings on route change so they don't accumulate after the
user has moved on.

### Why?

The redbox previously couldn't tell whether a route's instant warning
was raised while rendering it directly or while navigating into it — the
text and stack looked the same. And once one was surfaced, it stuck
around even after the user navigated elsewhere, where it was no longer
actionable. The two changes together make each warning describe the
phase that produced it and only stay visible while it's still relevant.

### Demo

- Subnav under Suspense:
[cookies](https://error-messages-overhaul-ibsl.labs.vercel.dev/scenario/41-subnav-cookies)
·
[fetch](https://error-messages-overhaul-ibsl.labs.vercel.dev/scenario/42-subnav-fetch)

### How

- New in-navigation variants of the body factory messages ("during the
initial render or a navigation") let the overlay tell nav warnings apart
from initial-render ones. The diagnostic clause stays aligned ("accessed
outside of \`<Suspense>\`"); only the phase and consequence change.
- The overlay reads the phase off the message, picks a matching
headline, and on route change drops nav warnings while keeping
initial-render errors in the redbox stack.
- Renames the \`GuidanceVariant\` value (\`'navigation' → 'dynamic'\`)
that collided with the new in-nav phase concept.

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

Move next.config evaluation inside the try catch where we import and
transform it

This ensures we print an appropriate help link if the export function
throws