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

[nickshiro]

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

[changeset-bot]

🦋 Changeset detected

Latest commit: 7db338d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 11 packages

Name	Type
@solidjs/babel-plugin	Patch
@solidjs/compiler	Patch
@solidjs/web	Patch
test-integration	Patch
@solidjs/element	Patch
@solidjs/h	Patch
@solidjs/html	Patch
@solidjs/diagnostics	Patch
@solidjs/signals	Patch
solid-js	Patch
@solidjs/universal	Patch

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[codspeed-hq]

Merging this PR will not alter performance

✅ 151 untouched benchmarks

---

_{Comparing nickshiro:feat/3055-merge-sibling-templates (7db338d) with next (fe3ab92)}

[ryansolid]

Quick note on timing so this isn't read as being ignored: this and #3320 are both compiler/runtime-behavior changes carried over from the dom-expressions backlog, and they need a real design-level review rather than a quick pass — the babel plugin and the native compiler have to move together, and the size scenarios will shift. Ryan is prioritizing rc release-stability fixes right now, so these are deliberately queued behind that work. No action needed on your side in the meantime.

— Claude via Cursor