feat(pdf): add PDF client, scraper, parser, and end-to-end tests

- Introduce PDF submodule with client, scraper, and generic parser
- Add filesystem PDF client and test-only mock routing
- Add end-to-end PDF scrape → parse tests with typed output
- Mirror HTML module architecture for consistency
- Expose PDF primitives via omniread public API

omniread/__init__.py

@@ -1,5 +1,6 @@
 from .core import Content, ContentType
 from .html import HTMLScraper, HTMLParser
+from .pdf import FileSystemPDFClient, PDFScraper, PDFParser
 
 __all__ = [
     # core
@@ -9,4 +10,9 @@ __all__ = [
     # html
     "HTMLScraper",
     "HTMLParser",
+
+    # pdf
+    "FileSystemPDFClient",
+    "PDFScraper",
+    "PDFParser",
 ]