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

tests/mocks/pdf/simple.pdf

@@ -0,0 +1,32 @@
+%PDF-1.4
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+<< /Type /Catalog /Pages 2 0 R >>
+endobj
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+<< /Type /Pages /Kids [3 0 R] /Count 1 >>
+endobj
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+<< /Type /Page /Parent 2 0 R /MediaBox [0 0 612 792] /Contents 4 0 R >>
+endobj
+4 0 obj
+<< /Length 44 >>
+stream
+BT
+/F1 12 Tf
+72 720 Td
+(Simple PDF Test) Tj
+ET
+endstream
+endobj
+xref
+0 5
+0000000000 65535 f
+0000000010 00000 n
+0000000061 00000 n
+0000000116 00000 n
+0000000203 00000 n
+trailer
+<< /Size 5 /Root 1 0 R >>
+startxref
+300
+%%EOF