feat(chunks): aggiunge validator.py — invarianti + metriche

Implementa validate() che controlla chunk_id duplicati, fence rotti,
size compliance (esclusi overflow) e calcola metriche aggregate.
Fix fixture cfg: min_chars=40 per allinearla ai chunk da 50 chars nel test.

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

chunks/validator.py

@@ -0,0 +1,63 @@
+from __future__ import annotations
+import re
+from chunks.models import ChunkingResult, Diagnostics
+from chunks.config import ChunkerConfig
+
+_OPEN_FENCE_RE = re.compile(r"^(`{3,}|~{3,})", re.MULTILINE)
+
+
+def _has_broken_fence(content: str) -> bool:
+    matches = _OPEN_FENCE_RE.findall(content)
+    return len(matches) % 2 != 0
+
+
+def validate(result: ChunkingResult, source: str, config: ChunkerConfig) -> Diagnostics:
+    errors: list[str] = []
+    warnings: list[str] = []
+    chunks = result.chunks
+
+    if not chunks:
+        warnings.append("Nessun chunk prodotto.")
+        return Diagnostics(errors=errors, warnings=warnings, metrics={"total_chunks": 0})
+
+    # chunk_id unici
+    seen_ids: set[str] = set()
+    for c in chunks:
+        if c.chunk_id in seen_ids:
+            errors.append(f"chunk_id duplicato: {c.chunk_id}")
+        seen_ids.add(c.chunk_id)
+
+    # fence rotto
+    for c in chunks:
+        if _has_broken_fence(c.content_original):
+            msg = f"fence rotto in {c.chunk_id} (righe {c.start_line}-{c.end_line})"
+            if config.fail_on_broken_fence:
+                errors.append(msg)
+            else:
+                warnings.append(msg)
+
+    # size compliance (esclusi overflow)
+    non_overflow = [c for c in chunks if not c.flags.get("is_overflow")]
+    for c in non_overflow:
+        if c.chars > config.max_chars:
+            errors.append(f"chunk {c.chunk_id} supera max_chars ({c.chars} > {config.max_chars})")
+
+    # metriche
+    total = len(chunks)
+    chars_list = [c.chars for c in chunks]
+    avg = sum(chars_list) // total if total else 0
+    compliant = sum(1 for c in non_overflow if config.min_chars <= c.chars <= config.max_chars)
+    compliance = round(compliant / len(non_overflow), 4) if non_overflow else 1.0
+
+    metrics = {
+        "total_chunks":     total,
+        "avg_chars":        avg,
+        "min_chars_actual": min(chars_list),
+        "max_chars_actual": max(chars_list),
+        "overflow_count":   sum(1 for c in chunks if c.flags.get("is_overflow")),
+        "sparse_count":     sum(1 for c in chunks if c.flags.get("is_sparse")),
+        "atomic_count":     sum(1 for c in chunks if c.content_type == "atomic_block"),
+        "size_compliance":  compliance,
+    }
+
+    return Diagnostics(errors=errors, warnings=warnings, metrics=metrics)