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_merged_block.py

@@ -0,0 +1,87 @@
+"""Test if merged blocks (TOC) fit in expanded_v."""
+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]
+
+# Extract and merge
+raw_blocks = pdf_mod.PDFTranslator()._extract_text_blocks(page)
+merged = pdf_mod.PDFTranslator()._merge_adjacent_blocks(raw_blocks, page.rect)
+
+# Find the TOC block (block 3 in our trace)
+toc = merged[3]
+print(f"TOC bbox: {toc['bbox']}")
+print(f"TOC font_size: {toc['font_size']}")
+print(f"TOC text length: {len(toc['text'])}")
+print(f"TOC text preview: {toc['text'][:80]!r}")
+
+# Redact
+toc_rect = fitz.Rect(toc['bbox'])
+page.add_redact_annot(toc_rect, fill=(1, 1, 1))
+page.apply_redactions(images=fitz.PDF_REDACT_IMAGE_NONE)
+
+# Test insert with expanded_v
+margin = 18
+page_rect = page.rect
+expanded_h = fitz.Rect(
+    max(toc_rect.x0, page_rect.x0 + margin),
+    toc_rect.y0,
+    page_rect.x1 - margin,
+    toc_rect.y1,
+)
+max_expand_y = min(
+    page_rect.y1 - margin - toc_rect.y1,
+    toc_rect.height * 3.0,
+)
+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}x{expanded_v.height}")
+
+# Translate the TOC text
+french_toc = """1. Introduction
+.......... 1
+2. Installation et configuration
+.......... 2
+3. Moteur de traduction
+.......... 3
+4. Préservation des formats
+.......... 4
+5. Assurance qualité
+.......... 5
+6. Performances et mise à l'échelle
+.......... 6
+7. Référence API
+.......... 7
+8. Dépannage
+.......... 8"""
+
+# Test on original
+rc1 = page.insert_textbox(
+    toc_rect, french_toc,
+    fontsize=12,
+    fontfile='C:/Windows/Fonts/arial.ttf',
+    align=fitz.TEXT_ALIGN_LEFT,
+    overlay=True,
+)
+print(f"Original rect: rc={rc1}")
+
+# Test on expanded_v
+rc2 = page.insert_textbox(
+    expanded_v, french_toc,
+    fontsize=12,
+    fontfile='C:/Windows/Fonts/arial.ttf',
+    align=fitz.TEXT_ALIGN_LEFT,
+    overlay=True,
+)
+print(f"expanded_v: rc={rc2}")
+
+doc.close()