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/debug_smart_fit.py

@@ -0,0 +1,81 @@
+"""Debug the smart-fit behavior on a real PDF."""
+import sys
+sys.path.insert(0, '.')
+import importlib.util
+
+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)
+
+import fitz
+
+# Open the test PDF
+doc = fitz.open("sample_files/test_corpus/test_pdf.pdf")
+page = doc[0]
+
+# Extract blocks
+translator = pdf_mod.PDFTranslator()
+raw_blocks = translator._extract_text_blocks(page)
+merged = translator._merge_adjacent_blocks(raw_blocks, page.rect)
+
+# Show block info
+for i, block in enumerate(merged[:5]):
+    print(f"Block {i}:")
+    print(f"  bbox: {block['bbox']}")
+    print(f"  font_size: {block['font_size']}")
+    print(f"  text: {block['text'][:50]!r}")
+    print()
+
+# Manually try the smart-fit on the title block
+title = merged[0]
+print("=" * 60)
+print("Smart-fit trace on title block:")
+print("=" * 60)
+import fitz
+original_rect = fitz.Rect(title["bbox"])
+page_rect = page.rect
+print(f"  original_rect: {original_rect}")
+print(f"  page_rect: {page_rect}")
+print(f"  page width: {page_rect.x1 - page_rect.x0}")
+print(f"  original width: {original_rect.width}, height: {original_rect.height}")
+
+# Simulate the expansion
+margin = 18
+max_x1 = page_rect.x1 - margin
+expanded_h = fitz.Rect(
+    max(original_rect.x0, page_rect.x0 + margin),
+    original_rect.y0,
+    max_x1,
+    original_rect.y1,
+)
+print(f"  expanded_h: {expanded_h}, width: {expanded_h.width}")
+
+next_block_y = page_rect.y1 - margin
+max_expand_y = min(
+    next_block_y - original_rect.y1,
+    original_rect.height * pdf_mod.MAX_VERTICAL_EXPANSION,
+)
+expanded_v = fitz.Rect(
+    expanded_h.x0,
+    expanded_h.y0,
+    expanded_h.x1,
+    expanded_h.y1 + max_expand_y,
+)
+print(f"  expanded_v: {expanded_v}, width: {expanded_v.width}, height: {expanded_v.height}")
+
+# Try the actual insert
+from fitz import TEXT_ALIGN_LEFT
+french_text = "Spécification technique : Office Translator v3.0"
+rc = page.insert_textbox(
+    expanded_v,
+    french_text,
+    fontsize=title["font_size"],
+    fontname="helv",
+    color=(0.1, 0.2, 0.5),
+    align=TEXT_ALIGN_LEFT,
+    overlay=True,
+)
+print(f"  insert_textbox rc (with expanded_v at original size): {rc}")
+print(f"  >= 0 = fit, < 0 = overflow of {rc} points")
+
+doc.close()