perf(web,babel-plugin,compiler): merge adjacent sibling templates - #3342

Summary

ref #3055

Merge adjacent fully static DOM elements in a JSX fragment into a single template.

Previously:

<>
  <div>First</div>
  <div>Last</div>
</>

created and cloned two independent templates. It now compiles to a single multi-root template:

  const template = _$template(
    `<div>First</div><div>Last</div>`,
    4
  );

  const view = template();

Implementation

• Detect maximal runs of adjacent, fully static DOM roots during fragment lowering.

• Combine those roots into one template while explicitly closing every non-void root.

• Preserve the external fragment shape by returning the cloned roots as a Node[].

• Add bit 4 to the internal template() flag:

  • 1: use document.importNode
  • 2: wrapped namespace template
  • 4: multi-root template

• Flags remain composable, so 5 represents a multi-root template that requires importNode.

• Keep the existing single-root cloning path unchanged and select the cloning strategy when the factory is created rather than branching for every clone.

• Implement the same lowering in both the Babel plugin and the native Oxc compiler.

• Support both generate: "dom" and generate: "dynamic" when the selected renderer is DOM.

The optimization is deliberately limited to safe static runs. Expressions, components, meaningful text, dynamic attributes, effects, refs, spreads, and wrapped SVG/MathML partials break the group. Hydratable
output is unchanged.

Added coverage for:

• adjacent static fragment roots;
• groups separated by an expression;
• dynamic roots remaining independent;
• adjacent void elements;
• combined importNode and multi-root flags;
• preservation of the fragment array shape;
• cloning each root on every factory call;
• HTML and SVG namespace preservation;
• Babel/Oxc output parity.

How did you test this change?

• Babel plugin: 255 tests passed
• Native compiler: 5360 tests passed, 28 skipped
• Rust feature matrix: 141 tests passed
• Web client: 738 tests passed
• Web server: 785 tests passed, 2 skipped
• Web hydration: 168 tests passed
• Web runtime through both Babel and Oxc: 9 tests passed in each mode
• Babel typecheck passed
• Web build and declaration generation passed
• Full workspace build passed