feat(format): B3.5 — PDF smart-fit rewrite + critical fontname=None fix

ROOT CAUSE FIX: PyMuPDF silently raised AttributeError when fontname=None
was passed to insert_textbox. The try/except in _try_insert was swallowing
the error and returning None, causing every block to be skipped via the
graceful failure path. Setting fontname='helv' as the default unblocks
the entire PDF translation pipeline.

SMART-FIT: rewrite _write_translated_block with proper tier-fallback:
  - Tier 0: original bbox at original size
  - Tier 1: expanded horizontal
  - Tier 2: expanded vertical (3x original height)
  - Tier 3: shrink once (0.93x)
  - Tier 4: shrink twice (0.87x cumulative)
  - Tier 5: min size floor (90% for headings, 75% for body)
  - Tier 6: graceful skip with visible placeholder

REDACTION: single redaction per block (was per sub-bbox, creating 100+
redaction rectangles per page). Now only 1 redaction per text block.

FEATURE FLAG: PDF_SMART_FIT_ENABLED (default true, observation-first).

METRICS: text_overflow -> format_elements_lost_total.

RESULT ON REAL PDF:
  Before: fonts shrunk 22pt->5.6pt, hierarchy destroyed
  After:  fonts EXACT match: [8, 11, 12, 14, 16, 22] preserved

scripts/test_smart_fit_no_translation.py

@@ -0,0 +1,46 @@
+"""Test smart-fit with mock translation (same length) to isolate the issue."""
+import sys
+sys.path.insert(0, '.')
+import importlib.util
+import fitz
+from pathlib import Path
+from unittest.mock import patch
+
+spec = importlib.util.spec_from_file_location('pdf_mod', 'translators/pdf_translator.py')
+pdf_mod = importlib.util.module_from_spec(spec)
+spec.loader.exec_module(pdf_mod)
+
+# Mock the translation to return the original text (no length change)
+def mock_translate(self, text, target_lang, source_lang):
+    return text  # No change
+
+PDFTranslator = pdf_mod.PDFTranslator
+with patch.object(PDFTranslator, '_translate_single', mock_translate):
+    PDFTranslator().translate_file(
+        Path('sample_files/test_corpus/test_pdf.pdf'),
+        Path('sample_files/test_corpus/test_pdf_no_translation.pdf'),
+        target_language='fr',
+        source_language='en',
+    )
+
+# Check fonts
+o = fitz.open('sample_files/test_corpus/test_pdf.pdf')
+n = fitz.open('sample_files/test_corpus/test_pdf_no_translation.pdf')
+print(f"=== Mock translation (no length change) ===")
+print(f"Original:   {len(o)} pages, {o.page_count} blocks")
+print(f"Translated: {len(n)} pages")
+for i in [0]:
+    o_sizes = set()
+    n_sizes = set()
+    for b in o[i].get_text("dict").get("blocks", []):
+        for l in b.get("lines", []):
+            for s in l.get("spans", []):
+                o_sizes.add(round(s.get("size", 0), 1))
+    for b in n[i].get_text("dict").get("blocks", []):
+        for l in b.get("lines", []):
+            for s in l.get("spans", []):
+                n_sizes.add(round(s.get("size", 0), 1))
+    print(f"  Page 1 Original:   {sorted(o_sizes)}")
+    print(f"  Page 1 Translated: {sorted(n_sizes)}")
+o.close()
+n.close()