Add 4 new tests to verify PNG and TIFF predictor functions use row-by-row
processing with bounded peak memory (2x stride), never pre-allocating full
output buffers inside tests.
- test_png_predictor_budget_enforcement_small_fixture: 200-byte fixture,
100-byte budget, verifies truncation at row boundary
- test_tiff_predictor_2_budget_enforcement_small_fixture: 160-byte fixture,
80-byte budget, verifies row-by-row processing for grayscale
- test_png_predictor_multiple_selectors_budget_per_row: 25-byte fixture
with all PNG selector types, verifies per-row budget checking
- test_tiff_predictor_2_rgb_budget_enforcement: 45-byte RGB fixture,
verifies multi-byte pixel handling with budget enforcement
All fixtures are under 250 bytes, no full-buffer pre-allocation, tests
mirror the row-by-row discipline from bf-49wmw production fix.
Closes bf-21hw8
319f81aaa3