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_with_redaction.py

@@ -0,0 +1,59 @@
+"""Test if redaction affects insert_textbox behavior."""
+import sys
+sys.path.insert(0, '.')
+import importlib.util
+import fitz
+
+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)
+
+doc = fitz.open('sample_files/test_corpus/test_pdf.pdf')
+page = doc[0]
+
+# Get the title block info
+raw_blocks = pdf_mod.PDFTranslator()._extract_text_blocks(page)
+title = raw_blocks[0]
+print(f"Title bbox: {title['bbox']}")
+print(f"Title text: {title['text']!r}")
+
+# Test 1: insert_textbox on fresh page
+french = "Spécification technique : Office Translator v3.0"
+rect = fitz.Rect(title['bbox'])
+rc1 = page.insert_textbox(
+    rect, french,
+    fontsize=22,
+    fontfile='C:/Windows/Fonts/arial.ttf',
+    color=(0.1, 0.2, 0.5),
+    align=fitz.TEXT_ALIGN_LEFT,
+    overlay=True,
+)
+print(f"Fresh page: rc={rc1} (negative = overflow, positive = fit)")
+
+# Test 2: Apply redaction, then insert_textbox
+page.add_redact_annot(rect, fill=(1, 1, 1))
+page.apply_redactions(images=fitz.PDF_REDACT_IMAGE_NONE)
+
+rc2 = page.insert_textbox(
+    rect, french,
+    fontsize=22,
+    fontfile='C:/Windows/Fonts/arial.ttf',
+    color=(0.1, 0.2, 0.5),
+    align=fitz.TEXT_ALIGN_LEFT,
+    overlay=True,
+)
+print(f"After redaction: rc={rc2}")
+
+# Test 3: With expanded_v
+expanded_v = fitz.Rect(rect.x0, rect.y0, rect.x1, rect.y1 + rect.height * 3)
+rc3 = page.insert_textbox(
+    expanded_v, french,
+    fontsize=22,
+    fontfile='C:/Windows/Fonts/arial.ttf',
+    color=(0.1, 0.2, 0.5),
+    align=fitz.TEXT_ALIGN_LEFT,
+    overlay=True,
+)
+print(f"After redaction, expanded_v ({expanded_v.width}x{expanded_v.height}): rc={rc3}")
+
+doc.close()