feat(core): memoize components by value of Element props

[rexlunae]

Closes #1929

Problem

When a component re-renders and passes an Element (e.g. children) to a child component, the Element is rebuilt from scratch on every render. VNode's PartialEq was pointer-based (Rc::ptr_eq), so the new allocation never compared equal to the old one — the child component re-rendered even when its props were completely unchanged.

fn app() -> Element {
    let mut show = use_signal(|| false);
    rsx!(
        div {
            p { "{show}" }
            Whatever { div { } } // rebuilt each render, new pointer
        }
    )
}

#[component]
fn Whatever(children: Element) -> Element {
    // re-ran on every app re-render, even though children never changed
    rsx!(div { {children} })
}

Change

• VNode's PartialEq now compares by value:
  • template compared by its content hash,
  • dynamic attributes compared by value,
  • dynamic nodes compared structurally (text, placeholders, fragments, and components — component props are compared with the same type-erased equality the renderer already uses for component memoization).
• The diff fast path in diff/node.rs keeps pointer equality (VNode::ptr_eq), since the diff algorithm walks the tree structurally and a value comparison there would be wasted work.
• The FragmentProps doc comment is updated: children are now memoized by value, so borrowing nodes from a parent no longer defeats memoization.

Tests

• tests/memoize_element_props.rs: a child component with unchanged children does not re-render across parent re-renders; it does re-render when the children's content changes.
• tests/diff_dynamic_node.rs (toggle_template, regression for bug: Panic on diff/node.rs #2815): updated — the first phase (parent re-render) now correctly produces no edits because the child is memoized; the placeholder/text toggling is still covered by driving the child scope directly.

All dioxus-core tests pass (including the fuzzing and kitchen-sink suites); clippy and fmt clean.

[rexlunae]

cc @ealmloff @jkelleyrtp — this makes Element props memoizable by value (issue #1929). I kept the diff fast path on pointer equality so the renderer's hot path is untouched.