The invisible text filter in serialize_page_text() was always recomputing
block text from spans, but when block.spans is empty (no span data available),
this produced empty text for all blocks. Added fallback to use pre-computed
block.text when span data is missing, maintaining backward compatibility.
Also added special case for figure blocks to always emit empty text regardless
of span data.
All 111 text module tests pass, including all invisible text filtering tests
for Tr=0-7 and include_invisible=true/false combinations.
Acceptance criteria PASS:
- rendering_mode 3 excluded by default: ✓
- rendering_mode 3 included when flagged: ✓
- Mixed block emits visible: ✓
- All-invisible block produces empty (no spurious \n\n): ✓
- Tr=4 treated same as Tr=3: ✓
Closes pdftract-38p8h
dddf81075f