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/vault_search_index.dart

@@ -10,9 +10,12 @@
 //                              pages,
 //   * the handwriting OCR text persisted in the sidecar's `ocrText` field.
 //
-// It does NOT (yet) index a PDF's embedded text layer — that is a known gap (see
-// the REPORT in the task / the class doc below). Matching uses the existing pure
-// search primitives (normalize / rank / snippet), so CJK substring search works.
+// It ALSO indexes a file-backed PDF's document body text, captured once at
+// import into the sidecar's `pageText` field by [PdfTextIndexer]: the embedded
+// (printed) text layer for a normal PDF, or background OCR of the rendered pages
+// for a RASTERIZED / scanned PDF that has no text layer. Matching uses the
+// existing pure search primitives (normalize / rank / snippet), so CJK substring
+// search works.
 
 import 'dart:io';
 
@@ -71,13 +74,14 @@ class VaultSearchHit {
 /// source of truth).
 ///
 /// HONEST SCOPE (what search covers / does NOT):
-///   * COVERS: note/doc titles, typed text boxes, and handwriting OCR text that
-///     has been persisted into a sidecar's `ocrText` field.
-///   * DOES NOT cover: a PDF's embedded (printed) text layer — only the user's
-///     annotations are indexed, not the underlying document body. Indexing the
-///     PDF text layer would require rendering each page through pdfrx at scan
-///     time; deferred. OCR is only present where it has already been run and
-///     written back to the sidecar.
+///   * COVERS: note/doc titles, typed text boxes, handwriting OCR text persisted
+///     into a sidecar's `ocrText` field, AND a PDF's document body text persisted
+///     into `pageText` at import — the embedded text layer, or background OCR of
+///     a rasterized/scanned PDF (see [PdfTextIndexer]).
+///   * CAVEAT: `pageText` is only present once import-time indexing has run and
+///     written it back to the sidecar. A PDF imported before this feature (or
+///     whose OCR backend was unavailable) has no `pageText`, so only its
+///     annotations are searchable until it is re-indexed.
 class VaultSearchIndex {
   VaultSearchIndex(this._vault);
 
@@ -174,6 +178,11 @@ class VaultSearchIndex {
       }
       final ocr = sidecar.ocrText;
       if (ocr != null && ocr.trim().isNotEmpty) parts.add(ocr.trim());
+      // Document body text captured at import: the PDF's embedded text layer,
+      // or background OCR of a rasterized/scanned PDF. Covers the underlying
+      // document, not just the user's annotations.
+      final body = sidecar.pageText;
+      if (body != null && body.trim().isNotEmpty) parts.add(body.trim());
     }
     return parts.join('\n');
   }