feat: integra pipeline PDF→Markdown a 9 stadi e test suite

Porta da main la riscrittura completa di conversione/_pipeline/ (9 stadi
PyMuPDF) e la suite tests/ senza modificare chunks/, step-8/, rag.py,
ollama/, retrieve.py, config.py.

requirements.txt: aggiunge PyMuPDF>=1.24.0 e pytest>=8.0, mantiene chromadb,
rimuove opendataloader-pdf e pymupdf4llm.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

conversione/_pipeline/__init__.py

@@ -0,0 +1,30 @@
+from .extract          import validate_pdf, extract_metadata
+from .structure        import analyze
+from .report           import build_report
+from .runner           import run
+from .validator        import validate
+from .models           import Block, Section, FontProfile
+from .stage1_metadata  import extract_raw_data
+from .stage2_layout    import analyze_layout
+from .stage3_font      import build_font_profile
+from .stage4_headers   import classify_blocks
+from .stage5_hierarchy import infer_hierarchy
+from .stage6_tree      import build_tree
+from .stage7_markdown  import serialize_tree
+from .stage8_normalize import normalize_hierarchy
+from .stage9_validate  import validate_markdown, ValidationResult
+
+__all__ = [
+    "validate_pdf", "extract_metadata",
+    "analyze", "build_report", "run", "validate",
+    "Block", "Section", "FontProfile",
+    "extract_raw_data",
+    "analyze_layout",
+    "build_font_profile",
+    "classify_blocks",
+    "infer_hierarchy",
+    "build_tree",
+    "serialize_tree",
+    "normalize_hierarchy",
+    "validate_markdown", "ValidationResult",
+]