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
