feat(view): stream deferred views

[petehunt]

Depends on #292 and reworks the #290 streaming-shell concept around the ordinary View type. Review the commit after a02c230 for the deferred-view diff.

Summary

• Add defer component() { placeholder } syntax and View::defer for manual futures.
• Store deferred work as a view part, so components, layouts, and nested deferred views compose normally without ShellView, include(), or finish().
• Stream the shell first and completed fragments in completion order; dropping the response body cancels pending work.
• Send inert annotated <template> patches and apply them as they arrive with the parser-blocking external defer_script() asset, with no inline scripts.
• Add focused coverage, formatter/docs updates, and a runnable deferred-view example.

This is more integrated than #290: it adds a deferred view-part variant, lets the router retain an owned request context while streaming, and makes futures-util a normal router dependency. In exchange, every view can participate and composition needs no parallel type or helper. Deferred tasks are one-shot; cloned views share the same task.

Testing

• cargo +nightly fmt --all
• cargo topcoat fmt (expected Leptos parse errors only)
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test --workspace --all-features
• RUSTDOCFLAGS="--cfg docsrs -Dwarnings" cargo +nightly doc --workspace --all-features --no-deps --locked
• cargo check -p deferred-view
• node --check crates/topcoat/browser/defer.js

Created by OpenAI Codex (GPT-5), which implemented and locally verified the change under maintainer direction.