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/models.py

@@ -0,0 +1,44 @@
+"""Strutture dati intermedie della pipeline: Block, Section, FontProfile."""
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+
+
+@dataclass
+class Block:
+    text: str
+    page: int
+    bbox: tuple[float, float, float, float]  # x0, y0, x1, y1
+    font_size: float
+    font_name: str
+    is_bold: bool
+    block_type: str = "paragraph"  # paragraph|header_candidate|list_item|table|ignore
+    space_before: float = 0.0
+    level: int = 0                 # assegnato da stage5 (0 = non header)
+    origin_spans: list[dict] = field(default_factory=list, repr=False)
+
+    @property
+    def x0(self) -> float: return self.bbox[0]
+    @property
+    def y0(self) -> float: return self.bbox[1]
+    @property
+    def x1(self) -> float: return self.bbox[2]
+    @property
+    def y1(self) -> float: return self.bbox[3]
+
+
+@dataclass
+class Section:
+    title: str
+    level: int           # 1, 2, 3
+    content: list[Block] = field(default_factory=list)
+    children: list[Section] = field(default_factory=list)
+    page_start: int = 0
+    source_block: Block | None = field(default=None, repr=False)
+
+
+@dataclass
+class FontProfile:
+    body_size: float
+    cluster_map: dict[float, int]   # font_size arrotondato → livello (1/2/3)
+    header_sizes: list[float]       # taglie candidate header, ordinate desc