B3.6 — fix two visual bugs reported on the user's prod PDF:
1. Title 'Spécification technique : Office Translator v3.0' overflowed
its 2-line bbox and overlapped the 'Version du document...' block.
Root cause: MAX_VERTICAL_EXPANSION was 1.5x the original height,
way too small for a long French title. Bumped to 6.0x.
2. 'Avis important' blue background had white rectangular patches.
Root cause: redaction always used fill=(1,1,1) (opaque white),
which erased the colored drawing underneath the text.
Fix: detect when a block's bbox intersects a page drawing,
and use fill=None (transparent) for the redaction in that case.
The original drawing survives intact.
B3.7 — eliminate remaining block-vs-next-block overlap:
Computes each block's 'next_block_y' (the y0 of the nearest block
below it on the same page) and uses it as the ceiling for vertical
expansion. Previously the smart-fit logic used the page bottom as
the ceiling, which let long translated blocks flow into their
neighbour (e.g. 'Pour la dernière version...' overlapping
'8. Résolution des problèmes' in the TOC).
Also includes:
- 9 new tests (6 B3.6 + 3 B3.7) — total 446 tests pass, zero regression
- scripts/verify_b3_6_fix.py — visual+structural verification
- Updated sample_files/test_corpus/test_pdf_translated.pdf with the
clean B3.6+B3.7 output
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
