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/rerun_translation.py

@@ -0,0 +1,33 @@
+"""Re-run translation with the new B3.5 code and save to disk."""
+import sys
+sys.path.insert(0, '.')
+import shutil
+from pathlib import Path
+import fitz
+
+src = Path('sample_files/test_corpus/test_pdf.pdf')
+dst = Path('sample_files/test_corpus/test_pdf_translated.pdf')
+
+from translators.pdf_translator import PDFTranslator
+PDFTranslator().translate_file(src, dst, target_language='fr', source_language='en')
+
+o = fitz.open(src)
+n = fitz.open(dst)
+print(f'Original:   {len(o)} pages, {src.stat().st_size} bytes')
+print(f'Translated: {len(n)} pages, {dst.stat().st_size} bytes')
+print()
+print('Page 1 font sizes:')
+o_sizes = set()
+n_sizes = set()
+for b in o[0].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[0].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('  Original:  ', sorted(o_sizes))
+print('  Translated:', sorted(n_sizes))
+o.close()
+n.close()