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_full_pipeline.py

@@ -0,0 +1,72 @@
+"""Test if the full pipeline (extract, redact, re-insert links) affects insert_textbox."""
+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]
+
+# Step 1: Extract blocks
+raw_blocks = pdf_mod.PDFTranslator()._extract_text_blocks(page)
+title = raw_blocks[0]
+print(f"Title bbox: {title['bbox']}")
+
+# Step 2: Simulate redaction
+title_rect = fitz.Rect(title['bbox'])
+page.add_redact_annot(title_rect, fill=(1, 1, 1))
+page.apply_redactions(images=fitz.PDF_REDACT_IMAGE_NONE)
+print("After redaction")
+
+# Step 3: Simulate link reinsertion (we have TOC links)
+links = list(page.get_links())
+print(f"Got {len(links)} links")
+for link in links:
+    from_rect = link.get("from")
+    if link.get("page") is not None:
+        page.insert_link({
+            "kind": fitz.LINK_GOTO,
+            "from": from_rect,
+            "page": link["page"],
+            "to": fitz.Point(72, 72),
+        })
+    elif link.get("uri"):
+        page.insert_link({
+            "kind": fitz.LINK_URI,
+            "from": from_rect,
+            "uri": link["uri"],
+        })
+print("After link reinsertion")
+
+# Step 4: NOW try insert_textbox
+french = "Spécification technique : Office Translator v3.0"
+rect = fitz.Rect(title['bbox'])
+expanded_v = fitz.Rect(rect.x0, rect.y0, rect.x1, rect.y1 + rect.height * 3)
+
+# Test on original 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"After full pipeline, original rect: rc={rc1}")
+
+# Test on expanded_v
+rc2 = 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 full pipeline, expanded_v: rc={rc2}")
+
+doc.close()