Feature: Add getPagesInfo() for batch page info retrieval (#20530)

[BellYun]

Description

This PR implements the getPagesInfo() method requested in #20530.
Currently, retrieving layout data for all pages requires calling getPage(i) sequentially, which causes N worker round-trips. This PR introduces a batch retrieval method that fetches view, rotate, and userUnit for all pages in a single round-trip.

Changes

• API: Added PDFDocumentProxy.prototype.getPagesInfo() in src/display/api.js.
• Worker: Added GetPagesInfo handler in src/core/worker.js.
• Tests: Added unit tests in test/unit/api_spec.js to ensure data consistency matches getPage().

Performance Verification

I measured the performance improvement using a benchmark script.
The structural improvement (N -> 1 round-trip) provides significant speedup, especially for large documents.

Document	Method	Time (avg)	Improvement
tracemonkey.pdf (14 pages)	getPage loop	1.5ms	-
	getPagesInfo (New)	0.9ms	1.67x Faster
pdf.pdf (1,310 pages)	getPage loop	83.3ms	-
	getPagesInfo (New)	44.1ms	1.89x Faster

Screenshot:

🔻 Click to see the benchmark logic

I verified the logic using a script similar to this (simplified for reproducibility in unit tests):

describe("getPagesInfo benchmark & verification", function () {
  const tracemonkeyFileName = "tracemonkey.pdf";

  describe("performance comparison", function () {
    let pdfLoadingTask, pdfDocument;

    beforeAll(async function () {
      pdfLoadingTask = getDocument(buildGetDocumentParams(tracemonkeyFileName));
      pdfDocument = await pdfLoadingTask.promise;
    });

    afterAll(async function () {
      await pdfLoadingTask.destroy();
    });

    it("should measure execution time vs multiple getPage calls", async function () {
      const numPages = pdfDocument.numPages;

      // Warm up
      await pdfDocument.getPagesInfo();

      // Method 1: Multiple getPage() calls
      const startGetPage = performance.now();
      const pageInfosOld = [];
      for (let i = 1; i <= numPages; i++) {
        const page = await pdfDocument.getPage(i);
        pageInfosOld.push({
          view: page.view,
          rotate: page.rotate,
          userUnit: page.userUnit,
        });
      }
      const endGetPage = performance.now();
      const getPageTime = endGetPage - startGetPage;

      // Method 2: Single getPagesInfo() call
      const startGetPagesInfo = performance.now();
      const pageInfosNew = await pdfDocument.getPagesInfo();
      const endGetPagesInfo = performance.now();
      const getPagesInfoTime = endGetPagesInfo - startGetPagesInfo;

      // Log results
      console.log(`\n--- Performance Results (${numPages} pages) ---`);
      console.log(`Multiple getPage(): ${getPageTime.toFixed(2)}ms`);
      console.log(`Single getPagesInfo(): ${getPagesInfoTime.toFixed(2)}ms`);
      console.log(`Speedup: ${(getPageTime / Math.max(getPagesInfoTime, 0.01)).toFixed(2)}x`);

      // Verify Consistency
      expect(pageInfosNew.length).toEqual(pageInfosOld.length);
      for (let i = 0; i < pageInfosNew.length; i++) {
        expect(pageInfosNew[i].view).toEqual(pageInfosOld[i].view);
      }
    });
  });

  describe("memory & stability", function () {
    it("should not show significant memory growth on repeated calls", async function () {
      const loadingTask = getDocument(buildGetDocumentParams(tracemonkeyFileName));
      const pdfDoc = await loadingTask.promise;
      
      const getMemory = () => (performance.memory ? performance.memory.usedJSHeapSize : 0);
      const startMem = getMemory();

      for (let i = 0; i < 20; i++) {
        await pdfDoc.getPagesInfo();
      }
      
      const endMem = getMemory();
      if (startMem > 0) {
         console.log(`Memory Growth: ${((endMem - startMem) / 1024 / 1024).toFixed(2)}MB`);
      }
      await loadingTask.destroy();
    });

    it("should handle concurrent calls", async function () {
      const loadingTask = getDocument(buildGetDocumentParams(tracemonkeyFileName));
      const pdfDoc = await loadingTask.promise;
      const results = await Promise.all([
        pdfDoc.getPagesInfo(),
        pdfDoc.getPagesInfo(),
        pdfDoc.getPagesInfo()
      ]);
      expect(results[0]).toEqual(results[1]);
      await loadingTask.destroy();
    });
  });
});