Retain interception route host slots (#97242)
## Summary
Interception routes represent a partial update to the layout that hosts
them. Today we model this for `children` by synthesizing a `__DEFAULT__`
route backed by `default-null`, but named siblings still use normal
default matching even though they should retain their active state too.
This makes retention relative to the interception host instead of the
`children` key. Every non-intercepting sibling gets the existing
`__DEFAULT__` marker backed by `default-null`, while slots inside the
newly selected interception subtree continue to use normal matching and
real defaults.
The coverage uses a named-only host to verify that named siblings retain
client state without evaluating a user default, that a sibling without a
default also retains, and that a hard refresh still loads the canonical
route.
## Example
Consider this route tree:
```text
app/
├── named-host/
│ ├── layout.tsx # renders canonical, content, secondary, and modal
│ ├── @canonical/
│ │ └── page.tsx
│ ├── @content/
│ │ ├── page.tsx # renders a stateful counter
│ │ └── default.tsx # throws if evaluated
│ ├── @secondary/
│ │ └── page.tsx # has no default.tsx
│ └── @modal/
│ ├── default.tsx
│ └── (..)named-target/
│ └── page.tsx # intercepted target
└── named-target/
└── page.tsx # canonical route
```
Before this change, a soft navigation from `/named-host` to
`/named-target` could replace `@content` with its throwing default and
treat `@secondary` as missing because only `children` had retain
semantics. After this change, only `@modal` switches to the intercepted
page; `@content` and `@secondary` keep their existing UI and the counter
keeps its state. A hard refresh still renders the canonical
`/named-target` route instead of the interception host.
## Verification
- Turbopack and webpack production coverage in
`interception-dynamic-segment`
- The same production coverage with Cache Components enabled
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