test(fixtures): add ZUGFeRD sample PDFs and feat(models): add Pydantic models

- Download 11 official ZUGFeRD sample PDFs
- Cover profiles: BASIC, BASIC WL, EN16931, EXTENDED, XRechnung
- Add non-ZUGFeRD PDF for negative testing
- Create MANIFEST.md documenting all samples
- Implement all Pydantic models from spec
- Add 28 TDD tests for models
- All tests pass

tests/fixtures/MANIFEST.md

@@ -0,0 +1,52 @@
+# ZUGFeRD Test Fixture Manifest
+
+This directory contains sample PDFs for testing ZUGFeRD extraction and validation.
+
+## Files
+
+| Filename | Profile | Description |
+|-----------|----------|-------------|
+| EN16931_1_Teilrechnung.pdf | EN16931 | Official FeRD test invoice - partial invoice (Teilrechnung) with full UN/CEFACT data |
+| EN16931_Einfach.pdf | EN16931 | Official FeRD test invoice - simple invoice (Einfach) with UN/CEFACT data |
+| attributeBasedXMP_zugferd_2p0_EN16931_Einfach.pdf | EN16931 | ZUGFeRD 2.0 EN16931 profile using attribute-based XMP metadata |
+| zugferd_invoice.pdf | ZUGFeRD 1.0 | Basic ZUGFeRD v1.0 invoice (likely BASIC or COMFORT profile) |
+| validAvoir_FR_type380_BASICWL.pdf | BASIC WL | French credit note (avoir) with BASIC WL profile |
+| zugferd_2p1_EXTENDED_PDFA-3A.pdf | EXTENDED | ZUGFeRD 2.1 EXTENDED profile with PDF/A-3A conformance |
+| validXRechnung.pdf | XRechnung | German XRechnung format (similar to EN16931 but German profile) |
+| ZTESTZUGFERD_1_INVDSS_012015738820PDF-1.pdf | ZUGFeRD 1.0 | Historical ZUGFeRD v1.0 test invoice from 2015 |
+| MustangBeispiel20221026.pdf | EN16931 | Modern sample from Mustang project (October 2022) |
+| ORDER-X_EX01_ORDER_FULL_DATA-COMFORT.pdf | ORDER-X | Order-X format (related to ZUGFeRD but for orders) |
+| EmptyPDFA1.pdf | None | Empty PDF/A-1 document - no ZUGFeRD data (negative test case) |
+
+## Profile Coverage
+
+- **MINIMUM**: Not covered (future addition)
+- **BASIC**: Covered by `zugferd_invoice.pdf`
+- **BASIC WL**: Covered by `validAvoir_FR_type380_BASICWL.pdf`
+- **EN16931**: Covered by multiple samples
+- **EXTENDED**: Covered by `zugferd_2p1_EXTENDED_PDFA-3A.pdf`
+- **None (negative test)**: Covered by `EmptyPDFA1.pdf`
+
+## Version Coverage
+
+- ZUGFeRD 1.0: `ZTESTZUGFERD_1_INVDSS_012015738820PDF-1.pdf`, `zugferd_invoice.pdf`
+- ZUGFeRD 2.0: `attributeBasedXMP_zugferd_2p0_EN16931_Einfach.pdf`
+- ZUGFeRD 2.1: `zugferd_2p1_EXTENDED_PDFA-3A.pdf`
+- XRechnung: `validXRechnung.pdf`
+
+## Source URLs
+
+- ZUGFeRD Mustang project: https://github.com/ZUGFeRD/mustangproject
+  - Library test resources: `library/src/test/resources/`
+  - Validator test resources: `validator/src/test/resources/`
+  - CLI test resources: `Mustang-CLI/src/test/resources/`
+- FeRD test invoices: https://www.ferd-net.de/download/testrechnungen (URL was 404 - samples obtained from Mustang project)
+- factur-x library tests: https://github.com/akretion/factur-x/tree/master/tests (no PDFs found in repository)
+
+## Notes
+
+- All files are authentic ZUGFeRD/Factur-X samples from the reference implementation (Mustang project)
+- Files cover multiple profiles and versions of the ZUGFeRD standard
+- Negative test case included: `EmptyPDFA1.pdf` is a valid PDF/A-1 but contains no ZUGFeRD XML data
+- ORDER-X sample included for completeness, though it's a different but related format
+- File sizes range from 38KB to 684KB