fix(verify): riconosce URL www. come terminatori validi + doc multi-documento

- _URL_TAIL ora matcha anche www.* (non solo https://) — evita falsi blockers
  su watermark tipo www.docsity.com
- README: documenta --collection / --stems per ingestion, retrieve e rag

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

chunks/verify_chunks.py

@@ -38,7 +38,7 @@ PUNCT_END = re.compile(
     r"|:$"    # introduzione a lista o formula
 )
 _HEX_END     = re.compile(r"[0-9a-fA-F]{8,}$")
-_URL_TAIL    = re.compile(r"https?://\S+(\s+\S+){0,3}$")  # URL con fino a 3 token extra
+_URL_TAIL    = re.compile(r"(https?://|www\.)\S+(\s+\S+){0,3}$")  # URL con fino a 3 token extra
 _MATH_SYMS   = re.compile(r"[∈∑≤≥≠∀∃∫√∞∂±×÷→←↔⊂⊃⊆⊇∩∪·°]")
 _ROMAN_END   = re.compile(r"\b(I{1,3}|IV|VI{0,3}|IX|XI{0,2}|XIV|XV|XVI{0,2}|XIX|XX{0,2})$")