feat(search): index PDF text, OCR scanned PDFs on import

Search now covers handwriting, the PDF text layer, AND scanned
(rasterized) PDFs.

- PdfTextIndexer runs at import: sums the embedded text layer across
  pages; if present it stores that as the document body, otherwise the
  PDF is rasterized and its rendered pages are OCR'd in the background.
  The result lands in the sidecar `pageText` field (distinct from
  `ocrText`, the handwriting OCR). Idempotent (skips a sidecar that
  already has pageText); degrades gracefully with no OCR engine.
- pdfrx_page_text_source abstracts text/render so it's testable.
- VaultSearchIndex now harvests title + typed text + handwriting OCR +
  PDF pageText, so search finds notes, typed PDFs and scanned PDFs.

analyze clean, 409 tests green.

lib/services/pdfrx_page_text_source.dart

@@ -0,0 +1,111 @@
+// lib/services/pdfrx_page_text_source.dart
+//
+// Production wiring for [PdfTextIndexer]'s two injected text sources, backed by
+// pdfrx (the same engine the editor renders with). Kept SEPARATE from
+// PdfTextIndexer so the indexer's logic (the threshold decision, idempotency,
+// sidecar persistence) is unit-testable without the native pdfium/OCR stack —
+// only this file touches pdfrx, dart:ui, and the OCR engine, and it is exercised
+// on-device, not in CI.
+
+import 'dart:async';
+import 'dart:typed_data';
+import 'dart:ui' as ui;
+
+import 'package:pdfrx/pdfrx.dart';
+
+import 'ocr_engine.dart';
+
+/// pdfrx-backed loaders for [PdfTextIndexer].
+class PdfrxPageTextSource {
+  const PdfrxPageTextSource._();
+
+  /// Load the embedded text layer of every page (page order). Each entry is a
+  /// page's raw text (possibly empty). Returns an empty list on any failure, so
+  /// the indexer treats the PDF as having no text layer (→ OCR fallback).
+  static Future<List<String>> loadEmbeddedText(String pdfPath) async {
+    PdfDocument? doc;
+    try {
+      doc = await PdfDocument.openFile(pdfPath);
+      final out = <String>[];
+      for (final page in doc.pages) {
+        final raw = await page.loadText();
+        out.add(raw?.fullText ?? '');
+      }
+      return out;
+    } catch (_) {
+      return const [];
+    } finally {
+      await doc?.dispose();
+    }
+  }
+
+  /// Render each page and OCR it (page order). Returns one entry per page
+  /// (empty where nothing was recognized), or an empty list when the PDF can't
+  /// be opened. Honours the OCR engine's own graceful no-op: when no backend is
+  /// available every page comes back empty.
+  ///
+  /// Rendering is done at [renderScale]× the page's native 72-dpi size to give
+  /// the recognizer enough resolution on scanned scans without exploding memory.
+  static Future<List<String>> ocrPages(
+    String pdfPath, {
+    double renderScale = 2.0,
+  }) async {
+    PdfDocument? doc;
+    try {
+      doc = await PdfDocument.openFile(pdfPath);
+      final out = <String>[];
+      for (final page in doc.pages) {
+        final text = await _ocrOnePage(page, renderScale);
+        out.add(text ?? '');
+      }
+      return out;
+    } catch (_) {
+      return const [];
+    } finally {
+      await doc?.dispose();
+    }
+  }
+
+  static Future<String?> _ocrOnePage(PdfPage page, double renderScale) async {
+    PdfImage? image;
+    try {
+      final fullWidth = page.width * renderScale;
+      final fullHeight = page.height * renderScale;
+      image = await page.render(
+        fullWidth: fullWidth,
+        fullHeight: fullHeight,
+      );
+      if (image == null) return null;
+      final png = await _bgraToPng(image.pixels, image.width, image.height);
+      if (png == null) return null;
+      return OcrEngine.recognizeImage(png);
+    } catch (_) {
+      return null;
+    } finally {
+      image?.dispose();
+    }
+  }
+
+  /// Encode pdfrx's BGRA8888 raw pixels as PNG (the format [OcrEngine] expects).
+  static Future<Uint8List?> _bgraToPng(
+    Uint8List bgra,
+    int width,
+    int height,
+  ) async {
+    final completer = Completer<ui.Image>();
+    ui.decodeImageFromPixels(
+      bgra,
+      width,
+      height,
+      ui.PixelFormat.bgra8888,
+      completer.complete,
+    );
+    final image = await completer.future;
+    try {
+      final data = await image.toByteData(format: ui.ImageByteFormat.png);
+      return data?.buffer.asUint8List();
+    } finally {
+      image.dispose();
+    }
+  }
+}