fix(document-processors): make glyph-corruption ratio of 0 disable the signal

Address round-4 review on PR #914:
- glyph_corruption_ratio <= 0 now disables the signal (previously `control_ratio
  > 0` fired on any single C0 control byte), matching the "0 disables" convention
  used elsewhere (document_max_pdf_size_mb) and the config comment. Add a
  zero-disables test.
- Correct the document_escalation_suppressed_total comment: corrupt_glyphs CAN
  appear there in the narrow case where structured is unregistered and OCR is
  registered-but-disabled (evaluate_escalation follows minimum="structured" past
  the missing rung to a gated-off OCR). Add a test for that suppressed decision.
- Add a test for the double-corruption edge: a structured re-extract that is also
  glyph-corrupt escalates structured->ocr with reason corrupt_glyphs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/unit/test_doc_classifier.py

@@ -387,6 +387,15 @@ def test_glyph_corruption_ratio_override_disables_trigger():
     assert "corrupt_glyphs" not in c.flags
 
 
+def test_glyph_corruption_ratio_zero_disables_trigger():
+    full = _GLYPH_CORRUPT
+    bounds = [{"page": 1, "start_offset": 0, "end_offset": len(full)}]
+    # 0 disables the signal (rather than firing on any single control byte).
+    c = clf.classify_from_text(full, bounds, glyph_corruption_ratio=0.0)
+    assert c.recommended_tier == "fast"
+    assert "corrupt_glyphs" not in c.flags
+
+
 def test_empty_doc_routes_ocr_not_structured():
     # Precedence: a scanned/empty doc (no text layer) has no control chars to leak,
     # so it must stay an OCR case, never structured.