feat(chunks): rilevamento universale has_math/has_table + dollarmath plugin

- parser.py: integra dollarmath_plugin (graceful fallback) — blocchi $$…$$ → token math_block atomico
- segmenter.py: gestisce math_block → Block(kind=math, atomic=True, plain_text=[formula])
- packer.py: has_table rileva anche <table> in blocchi html; has_math rileva $$ e \begin{ nel contenuto
- +11 test (parser×3, segmenter×3, packer×5) — totale 65 test

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

chunks/segmenter.py

@@ -191,6 +191,25 @@ def segment(tokens: list[Token], lines: list[str], config: ChunkerConfig) -> lis
             i += 1
             continue
 
+        # ── Math block (dollarmath plugin) ───────────────────────────────────
+        if tok.type == "math_block":
+            start = tok.map[0] if tok.map else 0
+            end = tok.map[1] if tok.map else start + 1
+            content = _lines_content(lines, start, end)
+            blocks.append(Block(
+                id=_make_id(),
+                kind="math",
+                content=content,
+                plain_text="[formula]",
+                atomic=True,
+                start_line=start,
+                end_line=end,
+                header_path=header_path,
+                chars=len(content),
+            ))
+            i += 1
+            continue
+
         # ── HTML block ────────────────────────────────────────────────────────
         if tok.type == "html_block":
             start = tok.map[0] if tok.map else 0