test: add tests for smart FREE behaviour and data preservation across lanes

Verifies frame-lanes.AC3.3, AC3.4, AC8.3:
- AC3.3: Smart FREE on shared frame frees only the lane if frame in use
- AC3.3: Smart FREE on last lane frees the frame and cleans up
- AC3.4: ALLOC unchanged — allocates fresh frame, assigns lane 0
- AC8.3: FREE on shared frame preserves other lanes' data

Added to tests/test_pe_lanes.py as TestSmartFree class with 4 test methods:
- test_free_on_shared_frame_preserves_other_lanes
- test_free_last_lane_returns_frame
- test_alloc_unchanged_allocates_fresh_frame
- test_data_preservation_across_free_lanes