Add `ContentHost` widget; use it for Xilem `AnyView`

[waywardmonkeys]

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Introduce masonry::widgets::ContentHost, a pass‑through single‑child container that hosts exactly one child and supports dynamic replacement.

In Xilem, we replace the bespoke DynWidget with ContentHost. AnyWidgetView now uses Pod<ContentHost>.

[waywardmonkeys]

Some other naming options:

• ContentSlot
• Slot
• MountPoint
• Mount
• HostBox
• Vessel
• Shell
• Envelope
• Cradle

[PoignardAzur]

What about Passthrough?

I like it!

Some other naming options

I think most of those evoke a more complex behavior than what this does.

[PoignardAzur]

(Quick notice: I might apply the recommended changes and merge this PR this weekend.)