chore: enhance PR template + add title validation workflow

[arthrod]

Summary

Implements pull request automation improvements:

1. Enhanced PR template — adds Summary, Type of Change, and Related Issues sections before the existing checklist, so contributors provide context alongside the verification checklist.

2. pr-validate.yml workflow — lightweight title gate that:

   • Rejects auto-generated Codex/… style titles
   • Enforces a minimum 10-character title length

Type of Change

• Documentation / chore

Related Issues

N/A

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Checklist

• pnpm typecheck
• pnpm lint:fix
• bun test
• pnpm brl
• pnpm changeset
• ui changelog

https://claude.ai/code/session_011PTTNeLZPvRTJF58SWfyU1

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[gemini-code-assist]

Code Review

This pull request executes a major architectural shift for the @platejs/pagination package, replacing the previous document-mutating engine with a derived projection pipeline that leverages @chenglou/pretext for hih-fidelity text measurement. The implementation introduces a structured layout process involving snapshotting, measurement, and composition, supported by a React-based overlay for continuous-view page breaks. Review feedback focuses on enhancing the performance and reliability of the DOM measurement logic, specifically recommending the reuse of a robust line-height resolution helper to prevent NaN errors and caching top-level block elements to eliminate O(N^2) complexity during measurement passes.

[gemini-code-assist]

Exporting resolveLineHeight allows it to be reused in other parts of the pagination plugin, such as the overlay renderer, ensuring consistent line-height resolution across measurement and rendering.

Suggested change

	function resolveLineHeight(style: CSSStyleDeclaration): number {
	export function resolveLineHeight(style: CSSStyleDeclaration): number {

[gemini-code-assist]

topLevelBlockElements is called for every block being measured. Since it iterates over all children of the editable, this results in O(N^2) complexity for a document with N blocks during a full measurement pass. Calling it once at the start of createDomMeasure and reusing the result improves performance significantly for large documents.

export function createDomMeasure(editable: HTMLElement): MeasureFn {
  const elements = topLevelBlockElements(editable);

  return (block) => {
    const dom = elements[block.path[0]];
    if (!dom) return null;

    const style = getComputedStyle(dom);
    const lineHeightPx = resolveLineHeight(style);

    return {
      boxSpacingPx: verticalBoxSpacing(style),
      heightPx: measureBlockHeight(
        block.text,
        resolveFont(style),
        contentWidth(dom, style),
        lineHeightPx
      ),
      lineHeightPx,
    };
  };
}

[gemini-code-assist]

Import resolveLineHeight to use it in the overlay renderer for robust line-height resolution.

Suggested change

	import { createDomMeasure, topLevelBlockElements } from './domMeasure';
	import { createDomMeasure, resolveLineHeight, topLevelBlockElements } from './domMeasure';

[gemini-code-assist]

Using Number.parseFloat on lineHeight can return NaN if the value is "normal", which results in a 0 line height. This would cause incorrect positioning if a block is split across pages (where lineStart > 0). Using the resolveLineHeight helper provides a robust fallback.

        const lineHeight = resolveLineHeight(getComputedStyle(el));