fix: derive dead render state without requiring socket

[mrdotb]

dead was computed as:

assigns[:socket] == nil or not LiveView.connected?(assigns[:socket])

so a call site that did not pass socket looked dead on every render, not just the first. With props diffing enabled that silently breaks updates:

• full_props? stays true, so data-props is resent -- but base_assigns is still filtered to changed keys, so the snapshot degrades to only the props that changed.
• props_diff: not full_props? stays false, so data-props-diff is never marked as changed and freezes at its mount value.

The client is in diff mode (data-use-diff="true"), reads only the frozen diff attribute, and the component silently stops reflecting updates. Nothing raises and nothing is logged.

A populated __changed__ means LiveView is re-rendering the component, which only happens in a connected view, so a render can only be dead on the first pass. That is also the only place dead is used for anything other than props: the SSR decision is already gated on init, so this leaves SSR behaviour byte-for-byte identical.

The existing props-diff tests could not catch this: they render the component directly and read the attributes off the HTML, where every attribute is always present. Marking only decides what LiveView puts on the wire. The new test asks the Rendered struct for its dynamic parts with change tracking enabled, so an attribute LiveView would skip comes back as nil -- and it fails without this change.

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