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_trace.py

@@ -0,0 +1,55 @@
+"""Debug trace the smart-fit behavior on a real PDF."""
+import sys
+sys.path.insert(0, '.')
+import importlib.util
+import fitz
+from pathlib import Path
+
+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)
+
+original_write = pdf_mod.PDFTranslator._write_translated_block
+
+def debug_write(self, page, block, font_path, is_rtl):
+    target_size = block['font_size']
+    original_rect = fitz.Rect(block['bbox'])
+    page_rect = page.rect
+    margin = 18
+    is_heading = target_size >= 14
+    if is_heading:
+        expanded_h = original_rect
+    else:
+        expanded_h = fitz.Rect(
+            max(original_rect.x0, page_rect.x0 + margin),
+            original_rect.y0,
+            page_rect.x1 - margin,
+            original_rect.y1,
+        )
+    next_block_y = page_rect.y1 - margin
+    max_expand_y = min(
+        next_block_y - original_rect.y1,
+        original_rect.height * 3.0,
+    )
+    expanded_v = fitz.Rect(
+        expanded_h.x0,
+        expanded_h.y0,
+        expanded_h.x1,
+        expanded_h.y1 + max_expand_y,
+    )
+    text = block['translated'][:40]
+    is_heading_str = "HEAD" if is_heading else "BODY"
+    print(f"  [{is_heading_str}] text={text!r} | orig={original_rect.width:.0f}x{original_rect.height:.0f} | exp_v={expanded_v.width:.0f}x{expanded_v.height:.0f} | size={target_size}", flush=True)
+    result = original_write(self, page, block, font_path, is_rtl)
+    print(f"      result: {result}", flush=True)
+    return result
+
+pdf_mod.PDFTranslator._write_translated_block = debug_write
+
+PDFTranslator = pdf_mod.PDFTranslator
+PDFTranslator().translate_file(
+    Path('sample_files/test_corpus/test_pdf.pdf'),
+    Path('sample_files/test_corpus/test_pdf_translated.pdf'),
+    target_language='fr',
+    source_language='en',
+)